MDEV-21976: Re-disable main.udf, reverts mistake in merge b30a0131

This test was disabled in a135f0ab, but
accidentally reverted during the merge in
https://github.com/MariaDB/server/compare/0186b0a07760...b30a01314217

Because it was a 3-way merge, the removal is not directly visible.
It can be verified manually though:

  Line not changed:
  git diff a135f0ab..0186b0a0 mysql-test/unstable-tests

  Line removed:
  git diff a135f0ab..b30a0131 mysql-test/unstable-tests


This commit fixes the Travis-CI build and makes the main 10.5 git branch
green again on Travis-CI (and whatever else build environment that also
is failing due to this test).

Related: MDEV-22592

mysql-test/unstable-tests

@@ -217,6 +217,7 @@ main.type_int                           : Modified in 10.4.12
 main.type_newdecimal                    : MDEV-20532 - Floating point differences
 main.type_ranges                        : MDEV-20532 - Floating point differences
 main.type_time                          : Modified in 10.4.12
+main.udf                                : MDEV-21976: Fails on Ubuntu Bionic 18.04, our Travis-CI platform
 main.union_crash-714                    : Modified in 10.4.12
 main.upgrade_MDEV-19650                 : Added in 10.4.13
 main.userstat                           : MDEV-12904 - SSL errors