Update voice count after user is blocked

[vladytimy]

When a user is blocked and his replies are deleted:

• _bbp_reply_count for a topic is updated
• _bbp_voice_count for a topic is not updated

This doesn't currently affect any functionality (some may wonder how is possible that a topic has 2 voices and 0 replies).
But 4577 proposes a patch that will make use of _bbp_voice_count to display a correct view of no-replies query.

[Clorith]

This is very likely a side-effect of recounts being disabled on the WordPress.org forums, due to performance issues with them, so this would likely either need a custom implementation, or the reworked recount system bbPress is working on implemented (preferably the latter)