Revert "Add hideSelectionFromResult function"

This reverts commit 84cf134a4e889e5ac85641bb2ac732b015911174.

New problems found in:
https://github.com/ivaynberg/select2/issues/2196

Original issue:
https://github.com/ivaynberg/select2/issues/1538

Pull request:
https://github.com/ivaynberg/select2/pull/2032

select2.js

@@ -2278,14 +2278,13 @@ the specific language governing permissions and limitations under the Apache Lic
 
         // single
         postprocessResults: function (data, initial, noHighlightUpdate) {
-            var selected = 0, selectedElm = null, self = this, showSearchInput = true;
+            var selected = 0, self = this, showSearchInput = true;
 
             // find the selected element in the result list
 
             this.findHighlightableChoices().each2(function (i, elm) {
                 if (equal(self.id(elm.data("select2-data")), self.opts.element.val())) {
                     selected = i;
-                    selectedElm = elm;
                     return false;
                 }
             });
@@ -2293,14 +2292,7 @@ the specific language governing permissions and limitations under the Apache Lic
             // and highlight it
             if (noHighlightUpdate !== false) {
                 if (initial === true && selected >= 0) {
-                    // By default, the selected item is displayed inside the result list from a single select
-                    // User can provide an implementation for 'hideSelectionFromResult' and hide it
-                    if(selectedElm !== null) {
-                        if(this.opts.hideSelectionFromResult(selectedElm))
-                            selectedElm.addClass("select2-selected");
-                    }
-                    else
-                        this.highlight(selected);
+                    this.highlight(selected);
                 } else {
                     this.highlight(0);
                 }
@@ -3029,14 +3021,9 @@ the specific language governing permissions and limitations under the Apache Lic
             choices.each2(function (i, choice) {
                 var id = self.id(choice.data("select2-data"));
                 if (indexOf(id, val) >= 0) {
-                    // By default, the selected item is hidden from the result list inside a multi select
-                    // User can provide an implementation for 'hideSelectionFromResult' and allow the same
-                    // element to be selected multiple times.
-                    if(self.opts.hideSelectionFromResult(choice) === undefined || self.opts.hideSelectionFromResult(choice)) {
-                        choice.addClass("select2-selected");
-                        // mark all children of the selected parent as selected
-                        choice.find(".select2-result-selectable").addClass("select2-selected");
-                    }
+                    choice.addClass("select2-selected");
+                    // mark all children of the selected parent as selected
+                    choice.find(".select2-result-selectable").addClass("select2-selected");
                 }
             });
 
@@ -3353,7 +3340,6 @@ the specific language governing permissions and limitations under the Apache Lic
         adaptContainerCssClass: function(c) { return c; },
         adaptDropdownCssClass: function(c) { return null; },
         nextSearchTerm: function(selectedObject, currentSearchTerm) { return undefined; },
-        hideSelectionFromResult: function(selectedObject) { return undefined; },
         searchInputPlaceholder: '',
         createSearchChoicePosition: 'top',
         shouldFocusInput: function (instance) {