[FIX] mass_mailing: cleanup 'assets_mail_themes_edition' sub-bundle

Solve SCSS compilation issues due to 'webclient.scss' that was initially
added in b3e7fd040b.
The fine is now unnecessary and it's actually causing the editor to
behave incorrectly.

closes odoo/odoo#72678

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/__manifest__.py

@@ -65,7 +65,6 @@
             ('include', 'web._assets_helpers'),
             'web/static/lib/bootstrap/scss/_variables.scss',
             'mass_mailing/static/src/scss/mass_mailing.ui.scss',
-            'web/static/src/legacy/scss/webclient.scss',
         ],
         'web.assets_common': [
             'mass_mailing/static/src/js/tours/**/*',