gdb: use objfile::pspace in objfile::unlink

I think it would make sense to use objfile::pspace instead of the
current program space here.  It reduces the risks of calling this
method with the wrong current program space set.

Change-Id: Id4f3644719f232640c83a1c7f4aa92eaa6af6c5c
Approved-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>