Constant BLOCK_REGION_NONE already defined in require_once()

When I install the Mongodb block module, after I have had block module deactivated, I get the error message Constant BLOCK_REGION_NONE already defined in require_once().

This was also reported here by @langworthy: http://drupal.org/node/1163584#comment-5496150.

We could have different approaches to this, here are three

1. Delete the constant defined by block module and define it again in the Mongodb block module
2. Delete the constant defined by block module and define our own
3. Keep the block module constant and define our own.

I do not like to keep things in the configuration that I do not use, and there could also be other modules that depends on checking BLOCK_REGION_NONE (I do not know if there are any, but anyhow, you could) so my personal vote goes to number one.