s390/ftrace: enforce DYNAMIC_FTRACE if FUNCTION_TRACER is selected

We have too many combinations for function tracing. Lets simply stick to
the most advanced option, so we don't have to care of other combinations.

This means we always select DYNAMIC_FTRACE if FUNCTION_TRACER is selected.

In the s390 Makefile also remove CONFIG_FTRACE_SYSCALLS since that
functionality got moved to architecture independent code in the meantime.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>