LibsolvSolver::GetResult(): Fix package update cases

* The transaction_type(.., ..., SOLVER_TRANSACTION_RPM_ONLY)
categorization of the transaction steps is incorrect since rpm
automatically replaces an older package version when a new one is
installed and thus such package removals would be omitted. Use a
simpler check instead.
* Omit the unnecessary explicit check whether a package really has to be
installed.

Thanks to Michael Schroeder @ SUSE for his help figuring this out.