cleanup develop-eggs

[gracinet]

develop-eggs is known to be the most frequent source of erratic stateful behaviour in buildout, to the point that some people actually propose to wipe it afresh at each bootstrap, but that's not currently the case.

I have a build that runs with an outdated version of anybox.recipe.openerp on exactly one buildslave (out of half a dozen), just because of this.

Since we're reboostraping at each build anyway, we should also remove develop-eggs at each build before the bootstrap recreates a new one for whatever legitimate use it may have.