arm64: Remove unused macros from assembler.h

Commit 68234df4

 ("arm64: kill flush_cache_all()") removed the
only users of these macros.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>