gdb: remove push_target free functions

gdb: remove push_target free functions

Same as the previous patch, but for the push_target functions.

The implementation of the move variant is moved to a new overload of
inferior::push_target.

gdb/ChangeLog:

	* target.h (push_target): Remove, update callers to use
	inferior::push_target.
	* target.c (push_target): Remove.
	* inferior.h (class inferior) <push_target>: New overload.

Change-Id: I5a95496666278b8f3965e5e8aecb76f54a97c185