Remove Asymptote support

[t-lenz]

We've discouraged using Asymptote for graphics for quite some time. We should finally remove support for it from our task format (replace deprecation notice by error message and remove unused code in cmsontrib/gerpythonformat/CommonConfig.py and Executable.py).

Current task statements with .asy images:

• tasks/ratatoskr - sample.asy not used => deleted
• tasks/bintree - done
• tasks/orks - done
• tasks-raw/beacon - done
• Tasks depending on Asymptote  #51

[chuyang-wang-dev]

WIP, see also branch (aioi.git) issues/47.