[FIX] web_editor: use valid properties for hovered custom outline button

Since [1] the CSS property reset for hovered outline buttons is using
invalid property values.

After this commit the color and image are only reset on non-hover:
- the `background-color` is already set to `none` by `.btn`,
- the `background-image` is only set if defined on the button's style -
there is no need to reset it.

[1]: https://github.com/odoo/odoo/commit/a010c91b5ee119cf54ed1a68a6ea06b2bc5f3978

task-2633169

closes odoo/odoo#86699

X-original-commit: 8530235bff30bfcad55cf39927f0733d30fe2eee
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/scss/web_editor.common.scss

@@ -793,15 +793,11 @@ blockquote {
 .btn-custom:hover, .btn-fill-custom:hover {
     filter: invert(0.2);
 }
-.btn-outline-custom {
-    &:not(:hover) {
-        background-color: transparent !important;
-        background-image: none !important;
-    }
-    &:hover {
-        background-color: '';
-        background-image: '';
-    }
+.btn-outline-custom:not(:hover) {
+    // Custom buttons have their fill color or gradient specified in their
+    // element style. They must only be shown on hover for outline buttons.
+    background-color: transparent !important;
+    background-image: none !important;
 }
 
 // Base snippet rules