gdb: remove find_target_at

... to make the current_inferior reference bubble up one level.  Since
find_target_at would become a trivial wrapper around
inferior::target_at, remove it.  I think it's clearer anyway to see
explicitly that the "subject" of the method call is the inferior.

Change-Id: I73ec44d37e7afea6e85b1689af65e32ae8e5a695
Approved-By: Tom Tromey <tom@tromey.com>