Remove plugin menu state tracking

It's no longer necessary, as Polymer 1.x's menu button disables
scrolling while a menu is open.

Removing the data binding also avoids the `Cannot use 'in' operator to
search for 'plugin' in undefined` that would occur when interacting with
the plugin menu.

src/gui/html/elements/loot-plugin-card.html

@@ -155,7 +155,7 @@ loot-clear-metadata
           <paper-tooltip for="loadsArchive">Loads Archive</paper-tooltip>
           <iron-icon id="hasUserEdits" icon="account-circle" hidden$="[[!hasUserEdits]]"></iron-icon>
           <paper-tooltip for="hasUserEdits">Has User Metadata</paper-tooltip>
-          <paper-menu-button id="menu" opened="{{data.isMenuOpen}}" horizontal-align="right">
+          <paper-menu-button id="menu" horizontal-align="right">
             <paper-icon-button icon="more-vert" class="dropdown-trigger"></paper-icon-button>
             <loot-menu class="dropdown-content">
               <paper-toggle-button id="showOnlyConflicts" checked="{{data.isConflictFilterChecked}}">Show Only Conflicts</paper-toggle-button>

src/gui/html/js/plugin.js

@@ -186,7 +186,6 @@
 
       /* UI state variables */
       this.id = this.name.replace(/\s+/g, '');
-      this.isMenuOpen = false;
       this._isEditorOpen = false;
       this.isConflictFilterChecked = false;
       this._isSearchResult = false;

src/tests/gui/html/js/test_plugin.js

@@ -256,12 +256,6 @@ describe('Plugin', () => {
       plugin.id.should.equal('testpluginname');
     });
 
-    it('should set isMenuOpen to false', () => {
-      const plugin = new loot.Plugin({ name: 'test' });
-
-      plugin.isMenuOpen.should.be.false();
-    });
-
     it('should set isEditorOpen to false', () => {
       const plugin = new loot.Plugin({ name: 'test' });