Fix focus halo on primary action button mixin

- Post new annotation button no longer renders blue halo when clicked with a mouse, and only when tabbed to with keyboard
- Remove `primary-action-btn.scss`- was not  used since a mixin served same purpose.

src/styles/mixins/forms.scss

+@use "../mixins/focus";
 @use "../variables" as var;
 
 /* Style input placeholders */
@@ -57,6 +58,7 @@
 }
 
 @mixin primary-action-btn {
+  @include focus.outline-on-keyboard-focus;
   color: var.$grey-1;
   background-color: var.$grey-mid;
   height: 35px;

src/styles/sidebar/components/focused-mode-header.scss

@@ -9,7 +9,6 @@
 
   &__btn {
     @include forms.primary-action-btn;
-    @include focus.outline-on-keyboard-focus;
     display: flex;
     margin-left: auto;
     height: 30px;

src/styles/sidebar/components/primary-action-btn.scss

-@use "../../mixins/forms";
-@use "../../variables" as var;
-
-// A dark grey button used for the primary action
-// in a form
-.primary-action-btn {
-  @include forms.primary-action-btn;
-}
-
-.primary-action-btn--short {
-  height: 30px;
-}
-
-.primary-action-btn__icon {
-  color: var.$grey-4;
-  display: inline-block;
-  font-weight: bold;
-  margin-left: -3px;
-  margin-right: 3px;
-  transform: translateY(1px);
-}

src/styles/sidebar/sidebar.scss

@@ -50,7 +50,6 @@
 @use './components/menu-section';
 @use './components/moderation-banner';
 @use './components/new-note';
-@use './components/primary-action-btn';
 @use './components/search-status-bar';
 @use './components/selection-tabs';
 @use './components/share-annotations-panel';