[Bug] priority mail did not query the priority mail network

[Mariculous]

See the PR
https://github.com/jamespetts/simutrans-extended/pull/369

The path explorer correctly explored the prioritymail network but express mail used the precalculated normal mail network to move around.
This effectively led to pure express mail routes being entirely ignored by any mail (except from very small amounts of mail. It is yet unclear to me how those few units made their way into pure priority mail routes)

This fix also comes with a slight code cleanup as it was caused by a mistaken conditional handling where no condition was needed at all.

[Vladki]

Thanks for finding that. I suspected that behavior long ago, but had no idea how to prove that.