[merge] don't force gc from bzr script

[Andrew Bennetts]

Martin Pool wrote:
> annotate says that Andrew recently added a gc call into the bzr
> script.  I can't find the review thread about it (though there
> probably was one).
> 
> Maybe this was just done to help him in debugging, and not intended to
> be merged?  I don't think we should keep it permanently because, as
> the comment above this paragraph says, it's measurably faster to skip
> garbage collection, and as it's not reliable we can't actually use it
> for any cleanup.

Oops!  Yes, this was a SNAFU by me.

BB:approve

-Andrew.