Remove linespec.c:symtab_collector class

Remove linespec.c:symtab_collector class

linespec.c has a symtab_collector class that is readily replaced by a
lambda.  I think the result is more clear.

I considered using the unordered_dense 'extract' method as well, to
make the code a bit more efficient -- but it wasn't clear to me if the
ordering of the vector mattered, and so I've just added a comment.

Reviewed-By: Guinevere Larsen <guinevere@redhat.com>