DmfsTask: remove legacy ical4android/DmfsTask.kt compatibility bridge #344

Part of #116. Depends on #343.

Analogous to: removal of LegacyAndroidEventBuilder, LegacyAndroidEventProcessor, and AndroidEvent class for calendar events.

The ical4android/DmfsTask.kt was deprecated in PR #182 as a temporary bridge. This issue removes it entirely.

synctools changes

• Delete ical4android/DmfsTask.kt
• Remove all references to the legacy class throughout the library
• Clean up transitional/legacy code in DmfsTaskBuilder and DmfsTaskProcessor (rename/remove any remaining "legacy" variants)

DAVx5 changes (separate PR)

• LocalTask.kt: remove all remaining references to ical4android.DmfsTask; the class now relies entirely on the new storage/tasks/DmfsTask (introduced in DmfsTask: finish new storage-level DmfsTask API #338)
• LocalTaskList.kt: same
• LocalTaskListStore.kt: same