s390: remove WARN_DYNAMIC_STACK

s390 is the only architecture which allows to set the
-mwarn-dynamicstack compile option. This however will also always
generate a warning with system call stack randomization, which uses
alloca to generate some random sized stack frame.

On the other hand Linus just enabled "-Werror" by default with commit
3fe617cc ("Enable '-Werror' by default for all kernel builds"),
which means compiles will always fail by default.

So instead of playing once again whack-a-mole for something which is
s390 specific, simply remove this option.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>