Cleanup of Alberta cmake module and source files

Summary

The FindAlberta cmake module is implemented in terms of pkg-config files. Those are provided by the Alberta library and contain all necessary information to link against the package.

Note, this is the second attempt to update this module, see !455 (merged) and !469 (closed) for two old merge-requests that were reverted or closed.

Details

In addition to the cmake cleanup, some source files are cleaned up, e.g., removing outdated documentation and removing global static variables.