Change remove_target_sections to method on program_space

Change remove_target_sections to method on program_space

This changes remove_target_sections to be a method on program_space.
This makes sense because this function manipulates data that is
attached to the program space.

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

	* progspace.h (struct program_space) <remove_target_sections>:
	Declare.
	* exec.c (program_space::remove_target_sections): Now a method.
	* exec.h (remove_target_sections): Don't declare.