gdb: remove current_top_target function

gdb: remove current_top_target function

The current_top_target function is a hidden dependency on the current
inferior.  Since I'd like to slowly move towards reducing our dependency
on the global current state, remove this function and make callers use

  current_inferior ()->top_target ()

There is no expected change in behavior, but this one step towards
making those callers use the inferior from their context, rather than
refer to the global current inferior.

gdb/ChangeLog:

	* target.h (current_top_target): Remove, make callers use the
	current inferior instead.
	* target.c (current_top_target): Remove.

Change-Id: Iccd457036f84466cdaa3865aa3f9339a24ea001d