gdb: remove arguments from inferior_created observable

gdb: remove arguments from inferior_created observable

I noticed that non of the listeners of the inferior_created observable
used either of the arguments.  Remove them.  This in turn allows
removing the target parameter of post_create_inferior.

Tested only by rebuilding.

gdb/ChangeLog:

	* observable.h <inferior_created>: Remove parameters.  Update all
	listeners.
	* inferior.h (post_create_inferior): Remove target parameter.
	Update all callers.

Change-Id: I8944cefdc4447ed5347dc927b75abf1e7a0e27e6