remove gsup_test

The test doesn't do much: it encodes an Insert Subscriber Data request for the
sole purpose to ensure the msgb is allocated large enough. A bug like that is
easily avoided statically.

Also, the lu functions will get refactored soon, it doesn't make sense to me to
drag this test along.

Change-Id: I42e1c72bf4cce8034f968cd4392773bf2b643c1b