t5510: cleanup remote in followRemoteHEAD dangling ref test

t5510: cleanup remote in followRemoteHEAD dangling ref test

A later patch will introduce a new test which closely mirrors this one.
Update this test to remove the 'custom-head' remote it creates.
Otherwise, the two tests will conflict with each other, as the second
one to execute will fail to create this remote (which already exists,
thanks to the first test).

Signed-off-by: Matt Hunter <m@lfurio.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>