Event tracking and logging: remove unused drag/drop events

[josephsl]

Hi,

Partially reverts #43:

Even though the add-on handles six UIA drag/drop events, only two (drag complete, drop target dropped) are actively handled (passive handling means only event info is logged and nothing else). Therefore drop other four events (drag start, drag cancel, drop target drag enter, drop target drag leave) from being tracked and logged - this involves removing these events from NVDA Core's UIA handler events map. These events may come back if they are found to be useful in various apps.

Thanks.