Removed deprecated `$color-mine-shaft` SASS variable

Migrate all usages to `$grey-6` (close approximate).

src/styles/mixins/forms.scss

@@ -112,6 +112,6 @@
   }
 
   &:hover:enabled {
-    background-color: var.$color-mine-shaft;
+    background-color: var.$grey-6;
   }
 }

src/styles/sidebar/components/annotation-publish-control.scss

@@ -24,7 +24,7 @@
   .annotation-publish-control__btn {
     $text-color: var.$grey-1;
     $default-background-color: var.$color-dove-gray;
-    $hover-background-color: var.$color-mine-shaft;
+    $hover-background-color: var.$grey-6;
     $h-padding: 9px;
     $height: 35px;
     $border-radius: 2px;

src/styles/variables.scss

@@ -21,7 +21,6 @@ $color-dove-gray: #626262;
 $gray-dark: #585858;
 
 $grey-6: #3f3f3f;
-$color-mine-shaft: #3a3a3a;
 
 $grey-7: #202020;