Administration: Remove obsolete placeholder styles.

Remove the browser-prefixed placeholder styling `-ms-input-placeholder` and custom opacity for `-moz-placeholder`. `--ms-input-placeholder` is no longer used by any supported browsers and Firefox now uses `color-mix` to set placeholder colors instead of reduced opacity.

Props afercia, abcd95, sabernhardt, aialvi, joedolson.
Fixes #63030.

git-svn-id: https://develop.svn.wordpress.org/trunk@60636 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson

src/wp-admin/css/color-picker.css

@@ -107,11 +107,6 @@
 
 .wp-color-picker::-moz-placeholder {
 	color: #646970;
-	opacity: 1;
-}
-
-.wp-color-picker:-ms-input-placeholder {
-	color: #646970;
 }
 
 .wp-picker-container input[type="text"].iris-error {

src/wp-admin/css/forms.css

@@ -227,11 +227,6 @@ textarea[readonly] {
 
 ::-moz-placeholder {
 	color: #646970;
-	opacity: 1;
-}
-
-:-ms-input-placeholder {
-	color: #646970;
 }
 
 .form-invalid .form-required,

src/wp-includes/css/media-views.css

@@ -155,11 +155,6 @@
 
 .media-frame ::-moz-placeholder {
 	color: #646970;
-	opacity: 1;
-}
-
-.media-frame :-ms-input-placeholder {
-	color: #646970;
 }
 
 /*