Drop support for .form-control-plaintext inside .input-group (#28972)

* Revert "Fix readonly-plain-text with button addon (#25871)"

This reverts commit ff40e00323.

* Add migration comment

scss/_input-group.scss

@@ -12,7 +12,6 @@
   width: 100%;
 
   > .form-control,
-  > .form-control-plaintext,
   > .custom-select,
   > .custom-file {
     position: relative; // For focus state's z-index

site/content/docs/4.3/migration.md

@@ -56,6 +56,7 @@ Changes to Reboot, typography, tables, and more.
 
 ## Forms
 
+- Dropped support for `.form-control-plaintext` inside `.input-group`
 - **Todo:** Move forms documentation to it's own top-level section
 - **Todo:** Rearrange source Sass files (under `scss/forms/`)
 - **Todo:** Combine native and custom checkboxes and radios