Remove some non-standard CSS properties

Author: agriyakhetarpal

assets/theme-css/code-highlight.css

@@ -306,7 +306,6 @@ html[data-theme="light"] {
   /* LineNumbersTable */
   .chroma .lnt {
     white-space: pre;
-    -webkit-user-select: none;
     user-select: none;
     margin-right: 0.4em;
     padding: 0 0.4em 0 0.4em;
@@ -315,7 +314,6 @@ html[data-theme="light"] {
   /* LineNumbers */
   .chroma .ln {
     white-space: pre;
-    -webkit-user-select: none;
     user-select: none;
     margin-right: 0.4em;
     padding: 0 0.4em 0 0.4em;

assets/theme-css/navbar.css

@@ -54,8 +54,6 @@ a.navbar-item.nav-external::after,
   font-family: inherit;
   font-size: 1em;
   padding: 0;
-  -moz-appearance: none;
-  -webkit-appearance: none;
   appearance: none;
   background: 0 0;
   border: none;

assets/theme-css/sphinx-design/_button.scss

@@ -23,9 +23,6 @@
     box-shadow 0.15s ease-in-out;
   vertical-align: middle;
   user-select: none;
-  -moz-user-select: none;
-  -ms-user-select: none;
-  -webkit-user-select: none;
 
   &:hover {
     text-decoration: none;