signup: Remove stale highlighting function.
signup: Remove stale highlighting function.

This function was introduced in
447dc0029a74c8ff47d13fa40ce97dc10f20b522; but we haven't been using
this highlighting feature; and the last control-group was removed from
the signl page in dc2bdb2aad622ed1c0560a22069e92252e42b0e4.

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