Don't change current program space in exec_target::close

Don't change current program space in exec_target::close

Now that we've removed the macros and moved various functions to be
methods on program_space (removing uses of current_program_space),
it's clear that exec_target::close can operate on program spaces
without changing the current program space.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* exec.c (exec_target::close): Don't change current program
	space.