Cleanup $dropdown SCSS variables

Cleanup $dropdown-* SCSS variables from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

$dropdown-width: 300px;
$dropdown-min-height: 40px;
$dropdown-max-height: 312px;
$dropdown-max-height-lg: 445px;
$dropdown-vertical-offset: 4px;
$dropdown-shadow-color: rgba(#000, 0.1);
$dropdown-title-btn-color: #bfbfbf;
$dropdown-loading-bg: rgba($white, 0.6);
$dropdown-toggle-active-border-color: darken($border-color, 14%);
$dropdown-fade-mask-height: 32px;
$dropdown-member-form-control-width: 163px;

1. Remove rgba() and darken() color function usage, use static values
2. Replace static color value #bfbfbf with GitLab UI design token or SCSS variable
3. Cleanup and remove any unused SCSS variables or styles