Unify dark and brand-secondary SCSS variables

We were using each one half the time, while they both had the same value
by default. It was impossible to know when me meant "use a dark color
here" or "use the secondary brand color" here.

So now we're only using one variable, so it's easy it'll be easy to add
CSS custom properties to overwrite this variable. We're choosing
`brand-secondary` because its name makes it less problematic to use a
light color.

app/assets/stylesheets/budgets/phases.scss

@@ -87,7 +87,7 @@
 
       &[aria-selected="true"],
       &.is-active {
-        @include background-with-text-contrast($dark);
+        @include background-with-text-contrast($brand-secondary);
         font-weight: normal;
       }
 
@@ -100,10 +100,6 @@
     &.current-phase-tab a {
       @include background-with-text-contrast($brand-secondary);
       padding-top: $line-height / 2;
-
-      &:hover {
-        background: $dark;
-      }
     }
   }