kselftest: arm64: Remove redundant clean target

The arm64 signal tests generate warnings during build since both they and
the toplevel lib.mk define a clean target:

Makefile:25: warning: overriding recipe for target 'clean'
../../lib.mk:126: warning: ignoring old recipe for target 'clean'

Since the inclusion of lib.mk is in the signal Makefile there is no
situation where this warning could be avoided so just remove the redundant
clean target.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200624104933.21125-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>