Remove the function armada_drm_vbl_event_remove_unlocked() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: NRickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>