move call for removeBackdrop() to backdrop()

Closes #11958 by rebasing & merging it

js/modal.js

@@ -145,7 +145,6 @@
     var that = this
     this.$element.hide()
     this.backdrop(function () {
-      that.removeBackdrop()
       that.$element.trigger('hidden.bs.modal')
     })
   }
@@ -156,6 +155,7 @@
   }
 
   Modal.prototype.backdrop = function (callback) {
+    var that = this
     var animate = this.$element.hasClass('fade') ? 'fade' : ''
 
     if (this.isShown && this.options.backdrop) {
@@ -186,11 +186,15 @@
     } else if (!this.isShown && this.$backdrop) {
       this.$backdrop.removeClass('in')
 
+      var callbackRemove = function() {
+        that.removeBackdrop()
+        callback && callback()
+      }
       $.support.transition && this.$element.hasClass('fade') ?
         this.$backdrop
-          .one($.support.transition.end, callback)
+          .one($.support.transition.end, callbackRemove)
           .emulateTransitionEnd(150) :
-        callback()
+        callbackRemove()
 
     } else if (callback) {
       callback()