gdb: remove gdbarch_displaced_step_fixup_p

gdb: remove gdbarch_displaced_step_fixup_p

The comment on the gdbarch_displaced_step_fixup gdbarch method
indicates that this method is optional and that GDB will perform some
default if this method is not supplied.  As such we define a predicate
gdbarch_displaced_step_fixup_p.

It may have been true at one point that the fixup method was optional,
but it is no longer true.  If this method is not defined and GDB tries
to complete a displaced step, then GDB is going to crash.

Additionally the gdbarch_displaced_step_fixup_p predicate is not used
anywhere in GDB.

In this commit I have removed the gdbarch_displaced_step_fixup_p
predicate, and I have updated the validation check for the
gdbarch_displaced_step_fixup method; if the
gdbarch_displaced_step_copy_insn method is defined then the fixup
method must also be defined.

I believe I've manually checked all the current places where
gdbarch_displaced_step_copy_insn is defined and they all also define
the fixup method, so this change should cause no problems for anyone.

There should be no user visible changes after this commit.

Approved-By: Pedro Alves <pedro@palves.net>