MDL-62930 theme_boost: Remove custom-select styles

These styles cause the custom select fields to overlap their containers. See the license field in the file picker single file info dialogue for an example.

theme/boost/scss/moodle/bs4alphacompat.scss

@@ -173,10 +173,6 @@ $tag-font-weight: bold !default;
     @include float-right();
 }
 
-.custom-select {
-    width: auto;
-}
-
 .fade.in {
     opacity: 1;
 }

theme/boost/style/moodle.css

@@ -8593,9 +8593,6 @@ a.dimmed_text:visited,
   #page-admin-report-backups-index .backup-unfinished[href]:focus, #page-admin-index .adminwarning.availableupdatesinfo .moodleupdateinfo.maturity50 .info.release[href]:focus {
     background-color: #c9302c; }
 
-.custom-select {
-  width: auto; }
-
 .fade.in {
   opacity: 1; }