fix: include the correct SCSS file for lti_block

lti_block has Sass for its display, but not for its editor.

During the `add_sass_to_fragment` refactoring, I mixed this up:
I added a non-existent scss file to the studio_view but didn't add
the actual scss file to the student_view.

Course authors using the (deprecated) lti_block saw:

    We're having trouble rendering your component
    Students will not be able to access this component. Re-edit your component to fix the error.

    Error: Sass not found: /edx/app/edxapp/edx-platform/xmodule/assets/LTIBlockEditor.scss

as a result of this bug.

Original PR: openedx#32592

Private-ref: https://2u-internal.atlassian.net/browse/TNL-11029

xmodule/lti_block.py

@@ -383,7 +383,6 @@ def studio_view(self, _context):
         fragment = Fragment(
             self.runtime.service(self, 'mako').render_cms_template(self.mako_template, context)
         )
-        add_sass_to_fragment(fragment, 'LTIBlockEditor.scss')
         add_webpack_js_to_fragment(fragment, 'LTIBlockEditor')
         shim_xmodule_js(fragment, self.studio_js_module_name)
         return fragment
@@ -499,6 +498,7 @@ def student_view(self, _context):
         """
         fragment = Fragment()
         fragment.add_content(self.runtime.service(self, 'mako').render_lms_template('lti.html', self.get_context()))
+        add_sass_to_fragment(fragment, 'LTIBlockDisplay.scss')
         add_webpack_js_to_fragment(fragment, 'LTIBlockDisplay')
         shim_xmodule_js(fragment, 'LTI')
         return fragment