s390/purgatory: remove duplicated build rule of kexec-purgatory.o

This is equivalent to the pattern rule in scripts/Makefile.build.

Having the dependency on $(obj)/purgatory.ro is enough.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220613170902.1775211-3-masahiroy@kernel.org


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>