feat!: Replace sass with vanilla css in custom tag block

xmodule/assets/CustomTagBlockEditor.scss

@@ -1,3 +0,0 @@
-.xmodule_edit.xmodule_CustomTagBlock {
-  @import "codemirror/codemirror.scss";
-}

xmodule/assets/codemirror/_codemirror.scss

@@ -1,5 +0,0 @@
-.CodeMirror {
-  background: #fff;
-  font-size: 13px;
-  color: #3c3c3c;
-}

xmodule/static/css-builtin-blocks/CustomTagBlockEditor.css

@@ -0,0 +1,5 @@
+.xmodule_edit.xmodule_CustomTagBlock .CodeMirror {
+    background: #fff;
+    font-size: 13px;
+    color: #3c3c3c;
+}

xmodule/template_block.py

@@ -9,7 +9,7 @@ from lxml import etree
 from web_fragments.fragment import Fragment
 from xmodule.editing_block import EditingMixin
 from xmodule.raw_block import RawMixin
-from xmodule.util.builtin_assets import add_webpack_js_to_fragment, add_sass_to_fragment
+from xmodule.util.builtin_assets import add_webpack_js_to_fragment, add_css_to_fragment
 from xmodule.x_module import (
     ResourceTemplates,
     shim_xmodule_js,
@@ -69,7 +69,7 @@ class CustomTagBlock(CustomTagTemplateBlock):  # pylint: disable=abstract-method
         fragment = Fragment(
             self.runtime.service(self, 'mako').render_cms_template(self.mako_template, self.get_context())
         )
-        add_sass_to_fragment(fragment, 'CustomTagBlockEditor.scss')
+        add_css_to_fragment(fragment, 'CustomTagBlockEditor.css')
         add_webpack_js_to_fragment(fragment, 'CustomTagBlockEditor')
         shim_xmodule_js(fragment, 'XMLEditingDescriptor')
         return fragment