Remove code to cope with LWPs wrapped as PIDs

Remove code to cope with LWPs wrapped as PIDs

Historically the Linux x86 watchpoint code did not cope with multi-
threaded processes and LWP IDs were passed to it wrapped as PIDs.
Not all entry points were converted when the Linux x86 watchpoint
code was made multi-thread-aware, so a handler was left in place to
cope with wrapped LWPs.  Since then all such entry points have been
converted to pass regular LWPs and the handler is now redundant.
This commit removes the handler and adds assertions to ensure no
wrapped LWPs are passed in future.

gdb/ChangeLog:

	* x86-linux-nat.c (x86_linux_dr_get, x86_linux_dr_set):
	Remove code to cope with LWPs wrapped as PIDs.
	Add assertions to ensure no wrapped LWPs are passed.