tdf#158961 Remove a z-index: 100 rule to keep Safari happy

Modules navigation was not showing up in Safari because it had too
few elements to overflow. For some reason the z-index: 100 rule for
the containing div triggered this buggy behaviour.

Change-Id: I50776401b567d125d086e826ec7a23694bdf6774
Reviewed-on: https://gerrit.libreoffice.org/c/help/+/161651
Tested-by: Jenkins
Reviewed-by: Juan José González <juanjosegzl@libreoffice.org>
Tested-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
(cherry picked from commit 7f6876711ed991c8cf83edcc47980639b58c2c7a)
Reviewed-on: https://gerrit.libreoffice.org/c/help/+/161647
(cherry picked from commit c903a5157e0b9fe44a624965b642d1af57ec2b28)
Reviewed-on: https://gerrit.libreoffice.org/c/help/+/161648
diff --git a/help3xsl/default.css b/help3xsl/default.css
index f01f567..54cf36d 100644
--- a/help3xsl/default.css
+++ b/help3xsl/default.css
@@ -680,7 +680,6 @@ li.disabled a {
.modules {
    border-bottom: 2px solid #f3f3f3;
    background-color: #233336;
    z-index: 100;
}
#modules:after, #langs:after {
    font-size: 30px;