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

Commit odoo@c3c98e9
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:
odoo@2274566

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
odoo@4dce6cc#diff-a50ffff96791f36c6dbe5c9273135e5aR783

closes odoo#39615

X-original-commit: 3ddc722
Signed-off-by: Quentin Smetz (qsm) <[email protected]>

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;
     }