remove DateDayBeforeMonth option

Remove undocumented config option that was made redundant via the
passthrough-all-jsgantt-options branch. Recommended by cloos at:

https://github.com/bestpractical/rt-extension-jsgantt/commit/fe3deb122d9a6bc358903b3c5f96ab875bdfdadd#comments
dispense with storing a diff of local changes to jsgantt.js in extension

A diff against the 3rd party code is derivable, so no need to store it in the
extension.
Version 1.02 releng
Merge branch 'remove-datedaybeforemonth-option'