gdb: remove lm_info_vector typedef

I think this typedef hinders readability.  First, it's not well named
(it's not clear it contains lm_info_target objects).  And hiding the
fact that it contains unique pointers is not very useful either.  I was
looking at the code in solib_target_current_sos where the unique
pointers get moved from the vector, and it wasn't obvious at all what
the source of the move was.

Change-Id: I4a5cda7c90554f018b7c466b1535b41d69cbcbe7
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>