landing_pages: Remove unwanted underline on link focus.
landing_pages: Remove unwanted underline on link focus.

When a user clicks on a link it triggers `:focus` and an
underline is displayed. This is not intentional.

Also, for keyboard users, we have a surrounding outline box
for focus, so this underline is redundant in that case too.

rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip