Introduce program_space::remove_objfile

Introduce program_space::remove_objfile

This introduces a new method, program_space::remove_objfile, and
changes the objfile destructor not to unlink an objfile from the
program space's list.

This is cleaner because, like the previous patch, it treats the
program space more like a container for objfiles.  Also, this makes it
possible to keep an objfile alive even though it has been unlinked
from the program space's list, which is important for processing in a
worker thread.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* progspace.h (struct program_space) <remove_objfile>: Declare.
	* progspace.c (program_space::remove_objfile): New method.
	* objfiles.c (unlink_objfile): Remove.
	(objfile::unlink): Call remove_objfile.
	(objfile): Don't call unlink_objfile.

Change-Id: I22f768827723dce21886fae9b3664532c8349e68