s390: drop meaningless 'targets' from tools Makefile

'targets' should be specified to include .*.cmd files to evaluate
if_changed or friends.

Here, facility-defs.h and dis-defs.h are generated by filechk.

Because filechk does not generate .*.cmd file, the 'targets' addition
is meaningless. The filechk correctly updates the target when its
content is changed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>