Remove "priority" and "max-speed"

[dkastl]

Is your feature request related to a problem? Please describe.
Starting from the apconfig.xml file users can specify priority and well as max-speed, but its purpose is unclear or even confusing, see for example #281 .
pgRouting is not an oppinionated library but leaves the decision to the user how to define the costs for example.

I can't see how priority and well as max-speed are used in the import process, except that they create additional columns. If thhey are used during import, then it should be clearly documented how, because otherwise we have a blackbox here.

If max-speed value is also imported from OSM data, then what happens with the value from the configuration file?

Describe the solution you'd like
Only import what's necessary and clearly defined. priority and well as max-speed can be removed in my opinion, because this can be later handled by the user.

Describe alternatives you've considered
Improve documentation.