Revert input group border changes (#24197)

Turns out #22612 is a bad idea and it makes all sorts of weird visual
glitches when working with form controls and transparent borders.

scss/_custom-forms.scss

@@ -139,7 +139,6 @@
   color: $custom-select-color;
   vertical-align: middle;
   background: $custom-select-bg $custom-select-indicator no-repeat right $custom-select-padding-x center;
-  background-clip: padding-box;
   background-size: $custom-select-bg-size;
   border: $custom-select-border-width solid $custom-select-border-color;
   @if $enable-rounded {
@@ -216,13 +215,11 @@
   z-index: 5;
   height: $custom-file-height;
   padding: $custom-file-padding-y $custom-file-padding-x;
-  overflow: hidden;
   line-height: $custom-file-line-height;
   color: $custom-file-color;
   pointer-events: none;
   user-select: none;
   background-color: $custom-file-bg;
-  background-clip: padding-box;
   border: $custom-file-border-width solid $custom-file-border-color;
   @include border-radius($custom-file-border-radius);
   @include box-shadow($custom-file-box-shadow);
@@ -235,8 +232,9 @@
 
   &::before {
     position: absolute;
-    top: 0;
-    right: 0;
+    top: -$custom-file-border-width;
+    right: -$custom-file-border-width;
+    bottom: -$custom-file-border-width;
     z-index: 6;
     display: block;
     height: $custom-file-height;
@@ -244,7 +242,8 @@
     line-height: $custom-file-line-height;
     color: $custom-file-button-color;
     background-color: $custom-file-button-bg;
-    border-left: $custom-file-border-width solid $custom-file-border-color;
+    border: $custom-file-border-width solid $custom-file-border-color;
+    @include border-radius(0 $custom-file-border-radius $custom-file-border-radius 0);
   }
 
   @each $lang, $text in map-get($custom-file-text, button-label) {

scss/_input-group.scss

@@ -72,7 +72,6 @@
   color: $input-group-addon-color;
   text-align: center;
   background-color: $input-group-addon-bg;
-  background-clip: padding-box;
   border: $input-btn-border-width solid $input-group-addon-border-color;
   @include border-radius($input-border-radius);
 
@@ -144,8 +143,6 @@
   // element above the siblings.
   > .btn {
     position: relative;
-    background-clip: padding-box;
-    border: $input-btn-border-width solid $input-group-btn-border-color;
 
     + .btn {
       margin-left: (-$input-btn-border-width);

scss/_variables.scss

@@ -371,7 +371,6 @@ $form-group-margin-bottom:       1rem !default;
 $input-group-addon-color:        $input-color !default;
 $input-group-addon-bg:           $gray-200 !default;
 $input-group-addon-border-color: $input-border-color !default;
-$input-group-btn-border-color:   $input-border-color !default;
 
 $custom-control-gutter:   1.5rem !default;
 $custom-control-spacer-y: .25rem !default;