WELCOME.md: remove usage statistics comments

Usage statistics functionality is no longer shipped with ImageJ.

This reverts commits:
* 7b6303ac88bbf7896a58edcf099c71e560a4845c.
* 49d6bc1f8491a4fe41101f88588895dc851041ad.
Bump parent pom version

Updated pom-scijava to version 2.47