AIX: Remove exec_one_dummy_insn hack

AIX: Remove exec_one_dummy_insn hack

Old AIX versions required GDB to update the stack pointer register and
execute at least one instruction before accessing the space newly allocated
on the user stack.  This was done using the exec_one_dummy_insn routine
in rs6000-nat.c

However, in currently supported AIX versions (tested on AIX 6.1), this hack
is no longer necessary.  In fact, removing the hack actually fixed several
test case failures, and removes a call to deprecated_insert_raw_breakpoint.

gdb/ChangeLog:

	* rs6000-nat.c (exec_one_dummy_insn): Remove.
	(store_register): Do not call exec_one_dummy_insn.