[FIX] web: 'text-muted' links, remove opacity-changes on hover

Remove exception initially introduced for 'o-btn-link-variant' mixin
when handling 'text-muted' links.

The opacity-change wasn't really needed and introduced inconsistencies
with other links designs and with the mixin logic.

In case of complex/custom scenarios that requires opacity-changes,
'o-opacity-x' utility-classes can be used to achieve the desired design.

Part of the overall v16 SCSS optimization/restyle, task-2704984.

task-2865713

Part-of: odoo/odoo#93049

addons/web/static/src/legacy/scss/utils.scss

@@ -174,7 +174,6 @@
     }
 
     &.text-muted, .text-muted {
-        @include o-hover-opacity;
         @include o-hover-text-color($default-color: $text-muted, $hover-color: $color-active);
     }
 }