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
stefanorigano (SRI)
d198ebbaf2