[FIX] website_event: repair hide_sponsors with correct selector

Before this fix, the 'hide_sponsors' value was used to hide the
row of sponsors in several views:
- page of all exhibitors
- page of single exhibitor
- if website_event_track is installed -> registration confirmation

But actually it did not do anything anymore, because the old
selector used to apply the display none !important was hard-coded
to the previous code structure. This is very unstable, and structure
changed first in 35f70e5f11

Also, o_wevent_sponsor_wrapper selector was not used anywhere.

FIX:

Use o_wevent_sponsor_wrapper to not depend of structure anymore.
'hide_sponsors' adds o_wevent_hide_sponsors on the view and the
sponsors bar will be hidden, as expected.

Task-3213873

closes odoo/odoo#115453

X-original-commit: 13778a454b10aa40fae3cc0e3b7fcbd9e8781ce1
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/website_event/static/src/scss/event_templates_page.scss

@@ -143,8 +143,6 @@
  * SPONSORS
  */
 
-// small hack to hide sponsors on specific views
-.o_wevent_hide_sponsors .container.mt32.mb16.d-none.d-md-block.d-print-none {
-    // Not a very accurate way to target the 'sponsors' block -> improve in master
+.o_wevent_hide_sponsors .o_wevent_sponsor_wrapper {
     display: none !important;
 }