[FIX] website: remove breaking z-index of parallax elements

Commit https://github.com/odoo/odoo/commit/c3c98e937e575853fe0b25fa6cb3c916e73fc591
introduced a z-index on parallax element to restrict the z-index -1 of
the s_parallax_bg in the snippets... this was however a bad idea as it
also restricts other positioned elements in there (like dropdown menus)
making them go under other elements in the page.

Fortunately, this z-index is in fact not needed at all since the
z-index -1 mentioned above has been removed with another commit:
https://github.com/odoo/odoo/commit/22745668cf66d802899dac526d810eba64b28c4b

However, the z-index -1 was necessary to force the s_parallax_bg under
static elements that go after it... and this was solved in 12.0 by
making those relative elements. See snippet refactoring in 12.0 with
https://github.com/odoo/odoo/commit/4dce6cc98b2b95a00249ac29245305e08f288e6c#diff-a50ffff96791f36c6dbe5c9273135e5aR783



closes odoo/odoo#39615

X-original-commit: 3ddc722f
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -917,7 +917,7 @@ table.table_desc tr td {
 .parallax {
     position: relative;
     overflow: hidden;
-    z-index: 0;
+
     > * {
         position: relative;
     }