Revert "MIPS: Add basic support for ptrace single step"

This reverts commit 7c86ff99

.

There are too many special cases for MIPS not covered by this patch.
In the end it might be better to implement single stepping in userland
than emulating it in the kernel.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>