gdb: remove unused fetch_inferior_event and inferior_event_handler parameters

I noticed that fetch_inferior_event receives the client_data parameter
from its caller, inferior_event_handler, but doesn't actually need it.
This patch removes it.  In turn, inferior_event_handler doesn't use its
parameter, so remove it too.

The `data` argument used when registering
remote_async_inferior_event_handler is changed to NULL, to avoid
confusion.  It could make people think that the value passed is used
somewhere, when in fact it's not.

gdb/ChangeLog:

	* inf-loop.c (inferior_event_handler): Remove client_data param.
	* inf-loop.h (inferior_event_handler): Likewise.
	* infcmd.c (step_1): Adjust.
	* infrun.c (proceed): Adjust.
	(fetch_inferior_event): Remove client_data param.
	(infrun_async_inferior_event_handler): Adjust.
	* infrun.h (fetch_inferior_event): Remove `void *` param.
	* linux-nat.c (handle_target_event): Adjust.
	* record-btrace.c (record_btrace_handle_async_inferior_event):
	Adjust.
	* record-full.c (record_full_async_inferior_event_handler):
	Adjust.
	* remote.c (remote_async_inferior_event_handler): Adjust.

Change-Id: I3c2aa1eb0ea3e0985df096660d2dcd794674f2ea