Remove unnecessary icon styles

These icon styles do not need to be defined in the plugin because
identical styles are already defined in Redmine itself, at least up to
version 6.1. Defining them globally in the plugin could even cause
unexpected UI issues. Removing these styles still allows icons (PNG or
SVG) to display correctly from Redmine 5.1 through trunk.

Author: hidakatsuya

assets/stylesheets/view_customize.css

@@ -69,10 +69,3 @@ input#view_customize_comments {
 .icon-view_customize-enable:not(:has(svg)) {
     background-image: url("../images/enable.png");
 }
-
-/* PNG icon support for Redmine 5.1 and earlier */
-.icon:not(:has(svg)) {
-  background-position: 0% 50%;
-  background-repeat: no-repeat;
-  padding-left: 20px;
-}