gdb: remove target_so_ops::free_so

target_so_ops::free_so is responsible for freeing the specific lm_info
object.  All implementations basically just call delete.  Remove that
method, make the destructor of lm_info virtual, and call delete directly
from the free_so function.  Make the sub-classes final, just because
it's good practice.

Change-Id: Iee1fd4861c75034a9e41a656add8ed8dfd8964ee
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>