genksyms: remove Makefile hack

This workaround was introduced for suppressing the reduce/reduce conflict
warnings because the %expect-rr directive, which is applicable only to GLR
parsers, cannot be used for genksyms.

Since there are no longer any conflicts, this Makefile hack is now
unnecessary.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>