gdb: fix some inferior_created observer signatures

gdb: fix some inferior_created observer signatures

I just noticed that in a7aba2668a7b ("gdb: remove arguments from
inferior_created observable"), I forgot to update
aix_thread_inferior_created and procfs_inferior_created, which are in
files I can't compile.

Remove the parameters from aix_thread_inferior_created.  And simply
remove procfs_inferior_created, since it's empty anyway.

gdb/ChangeLog:

	* aix-thread.c (aix_thread_inferior_created): Remove parameters.
	* procfs.c (procfs_inferior_created): Remove.
	(_initialize_procfs): Don't register procfs_inferior_created.

Change-Id: Ifc7def7c096332033b5d466d32cb873d1df18c2c