Remove some unnecessary inferior_ptid setting/restoring when fetching/storing registers

Remove some unnecessary inferior_ptid setting/restoring when fetching/storing registers

Now that the to_fetch_registers, to_store_registers and
to_prepare_to_store target methods don't rely on the value of
inferior_ptid anymore, we can remove a bunch of now unnecessary setting
and restoring of inferior_ptid.

The asserts added recently in target_fetch_registers and
target_store_registers, which validate that inferior_ptid matches the
regcache's ptid, must go away.  It's the whole point of this effort, to
not require inferior_ptid to have a particular value when calling these
functions.

One thing that I noticed is how sol-thread.c's ps_lgetregs and friends
use the current value of inferior_ptid instead of what's passed as
argument (ph->ptid), unlike proc-service.c's versions of the same
functions.  Is it expected?  I left it like this in the current patch,
but unless there's a good reason for it to be that way, I guess we
should make it use the parameter.

gdb/ChangeLog:

	* fbsd-tdep.c (fbsd_corefile_thread): Don't set/restore
	inferior_ptid.
	* proc-service.c (ps_lgetregs, ps_lsetregs, ps_lgetfpregs,
	ps_lsetfpregs): Likewise.
	* regcache.c (regcache_raw_update, regcache_raw_write): Likewise.
	* sol-thread.c (ps_lgetregs, ps_lsetregs, ps_lgetfpregs,
	ps_lsetfpregs): Likewise.
	* target.c (target_fetch_registers, target_store_registers):
	Remove asserts.