remove unneeded aria-describedby and aria-hidden attrs on studio modal

cms/templates/js/basic-modal.underscore

 <div class="wrapper wrapper-modal-window wrapper-modal-window-<%= name %>"
-     aria-describedby="modal-window-description"
      aria-labelledby="modal-window-title"
-     aria-hidden=""
      role="dialog">
     <div class="modal-window-overlay"></div>
     <div class="modal-window <%= viewSpecificClasses %> modal-<%= size %> modal-type-<%= type %>" tabindex="-1" aria-labelledby="modal-window-title">

cms/templates/ux/reference/modal_access-component.html

-<div class="wrapper wrapper-modal-window wrapper-modal-window-edit-xblock" aria-describedby="modal-window-description" aria-labelledby="modal-window-title" aria-hidden="" role="dialog">
+<div class="wrapper wrapper-modal-window wrapper-modal-window-edit-xblock"  aria-labelledby="modal-window-title" role="dialog">
   <div class="modal-window-overlay"></div>
   <div class="modal-window confirm modal-med modal-type-html modal-editor" style="top: 50px; left: 400px;" tabindex="-1" aria-labelledby="modal-window-title">
     <div class="edit-xblock-modal">

cms/templates/ux/reference/modal_bulkpublish-section.html

 <%! from django.utils.translation import ugettext as _ %>
 
-<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-section" aria-describedby="modal-window-description" aria-labelledby="modal-window-title" aria-hidden="" role="dialog">
+<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-section" aria-labelledby="modal-window-title" role="dialog">
     <div class="modal-window-overlay"></div>
     <div style="top: 5%; left: 30%;" class="modal-window confirm modal-med modal-type-confirm">
         <div class="bulkpublish-section-modal">

cms/templates/ux/reference/modal_bulkpublish-subsection.html

 <%! from django.utils.translation import ugettext as _ %>
 
-<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-subsection" aria-describedby="modal-window-description" aria-labelledby="modal-window-title" aria-hidden="" role="dialog">
+<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-subsection" aria-labelledby="modal-window-title" role="dialog">
     <div class="modal-window-overlay"></div>
     <div style="top: 5%; left: 30%;" class="modal-window confirm modal-med modal-type-confirm">
         <div class="bulkpublish-subsection-modal">

cms/templates/ux/reference/modal_bulkpublish-unit.html

 <%! from django.utils.translation import ugettext as _ %>
 
-<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-unit" aria-describedby="modal-window-description" aria-labelledby="modal-window-title" aria-hidden="" role="dialog">
+<div class="wrapper wrapper-modal-window wrapper-modal-window-bulkpublish-unit" aria-labelledby="modal-window-title"  role="dialog">
     <div class="modal-window-overlay"></div>
     <div style="top: 5%; left: 30%;" class="modal-window confirm modal-med modal-type-confirm">
         <div class="bulkpublish-unit-modal">