Remove unused variables

[Quy]

These variables are defined, but not utilized. Remove all or advise which ones to keep.

$inverse-bg
$inverse-color
$font-size-xs
$abbr-border-color
$btn-toolbar-margin
$custom-control-indicator-margin-y
$zindex-navbar
$navbar-border-radius
$navbar-brand-padding-y
$nav-item-margin
$nav-item-inline-spacer
$nav-tabs-justified-link-border-color
$nav-tabs-justified-active-link-border-color
$progress-box-shadow
$pre-bg
$pre-border-color

[Luneya]

$font-size-xs is still used in variables.scss