Remove the possibility of passing the mod-file as a string in the first shell argument

This feature is ill-designed and no longer needed by the GUI. And is not very
useful: it is not possible to interact with the preprocessor without using the
filesystem, since the preprocessor creates many files anyways.

If we really need to reimplement such a feature, it should rather be redesigned
by reading the modfile from standard input (cin). That could be triggered by
using "-" as the filename argument (as is already done by several standard Unix
utilities).