Remove comment from versions.cfg that became invalid in zc.buildout 2.2.4

After upgrading `zc.buildout` to `2.2.4` our buildout fails with:

```
mr.developer: Queued 'opengever.maintenance' for checkout.
mr.developer: Skipped checkout of existing package 'opengever.maintenance'.
Getting distribution for 'zc.buildout'.
Got zc.buildout 2.2.4.
Upgraded:
  zc.buildout version 2.2.4;
restarting.
Generated script '/Users/lukasgraf/Plone/buildouts/opengever/og-core-4.x/bin/buildout'.
The constraint, 1.21 # We need at least 1.18 for the git rev option, is not consistent with the requirement, 'mr.developer==1.21'.
While:
  Installing.
  Loading extensions.
Error: Bad constraint 1.21 # We need at least 1.18 for the git rev option mr.developer==1.21
```

I don't think comments on the same line were ever officially allowed in buildout configs, and since `2.2.4` they definitely seem to break things.

I therefore removed the comment entirely since we've been using a recent version of `mr.developer` for a long time anyway.

@phgross