gdb/python: add unlink () method to gdb.Objfile object

gdb/python: add unlink () method to gdb.Objfile object

This commit adds method allowing one remove any objfile. This is meant
to be used to remove objfiles for dynamic code when this dynamic code
is discarded. However gdb.Objfile.unlink() makes no attempt to ensure
this - to make it consistent with other Python API to create and modify
objfiles related structures (compunits, symbol tables and so on).

Reviewed-By: Eli Zaretskii <eliz@gnu.org>