kbuild: rpm-pkg: delete firmware_install to fix build error

Commit 5620a0d1 ("firmware: delete in-kernel firmware") deleted
in-kernel firmware support, including "make firmware_install".

Since then, "make rpm-pkg" / "make binrpm-pkg" fails to build with
the error:

  make[2]: *** No rule to make target `firmware_install'.  Stop.

Commit df85b2d7 ("firmware: Restore support for built-in firmware")
restored the build infrastructure for CONFIG_EXTRA_FIRMWARE, but this
is out of the scope of "make firmware_install".  So, the right thing to
do is to kill the use of "make firmware_install".

Fixes: 5620a0d1 ("firmware: delete in-kernel firmware")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>