[Ref] Unify instrument extension writing code for module and instrument formats, and remove the old macro-based cruft.

[Imp] Like in in IT/XM, unneeded instrument properties are no longer written to ITI/XI files as well.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@22571 56274372-70c3-4bfc-bfc3-4c3a0b034d27

Author: sagamusix