pc-bios/s390-ccw: Use rm command during make clean

This reverts a change that replaced the "rm -f" command with the
undefined variable RM (expected to be set by make), and causes the
"make clean" command to fail for a s390 target:

make[1]: Entering directory '/usr/src/qemu/build/pc-bios/s390-ccw'
rm -f *.timestamp
*.o *.d *.img *.elf *~ *.a
/bin/sh: *.o: command not found
Makefile:39: recipe for target 'clean' failed
make[1]: *** [clean] Error 127
make[1]: Leaving directory '/usr/src/qemu/build/pc-bios/s390-ccw'
Makefile:489: recipe for target 'clean' failed
make: *** [clean] Error 1

Fixes: 3e4415a7 ("pc-bios/s390-ccw: Add core files for the network
                      bootloading program")
Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
Message-Id: <20170814204450.24118-2-farman@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>