implicitclass: Do not check availability of "gs" and "pdftops" executables

implicitclass: Do not check availability of "gs" and "pdftops" executables

Instead, check by the presence of "gstoraster" and "pdftoraster"
filters whether we have configured cups-filters for Ghostscript and/or
Poppler use.

Checking for the presence of an executable is too comlex, as we would
need to go through the PATH environment variable.