GDB: testsuite: s390: Don't return value from top-level (sed)

This patch is purely the result of running:

$ sed -i 's/^    return -1/    return/' s390*.exp*

and

$ sed -i 's/^    return 0/    return/' s390*.exp*

inside gdb/testsuite/gdb.arch.

I didn't include changes made to return statements inside procedures.

Approved-By: Tom Tromey <tom@tromey.com>