gdb: linux-nat: make linux_nat_filter_event return void

I noticed that linux_nat_filter_event returns a value, but its caller
doesn't use it.  This has been since 9c02b525 ("linux-nat.c: better
starvation avoidance, handle non-stop mode too").  Before that commit,
the return value was used to tell the caller whether to continue
processing that event or not.  But since then, the model is that we pull
all events from the kernel and linux_nat_filter_event just saves the
status to the lwp_info structure if it thinks it's relevant.  And the
caller, linux_nat_wait_1, selects a status at random amongst the threads
with a pending status.  So essentially, the return value of
linux_nat_filter_event does not have a reason to be anymore.  Change it
so it returns void.

gdb/ChangeLog:

	* linux-nat.c (linux_nat_filter_event): Return void.

Change-Id: I35662868910f5122772ed92a512adfbf4da12d87