Change varobj.c:rootlist to a std::list

Change varobj.c:rootlist to a std::list

This changes varobj.c:rootlist to be a std::list.  This lets us remove
some code.  std::list is chosen because its iterator invalidation
approach suits the all_root_varobjs API.

I considered replacing all_root_varobjs with "external iteration", but
haven't gotten around to doing so.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* varobj.c (struct varobj_root) <next>: Remove.
	(struct vlist): Remove.
	(rootlist): Now a std::list.
	(install_variable, uninstall_variable, all_root_varobjs): Update.