Update MAINTAINERS.txt to reflect reality for update.module

I kind of hate to do this, but after a brief chat with merlinofchaos in IRC, we've come to the conclusion that merlinofchaos is completely out of the loop on update status module, and really shouldn't be considered a maintainer of it in core any more:

[1:11pm] merlinofchaos: dww: No, it's so far beneath my radar I"ve almost forgotten it exists.
[1:12pm] dww: merlinofchaos: should we patch MAINTAINERS.txt to remove you in D7? ;)
[1:12pm] merlinofchaos: dww: Probably a good idea.