Revert "Do not duplicate find-links already present in stack/slapos.cfg ."

This reverts commit 781c1a4441a93f53bd694c4659a8d3b088b7c9f6.
Extended .cfg files are not cooperating, so += is unusable in [buildout]
section, even for find-links.

Conflicts:
	stack/erp5/buildout.cfg
(cherry picked from commit b4085e5859f6ea257dfc0ac7befa6ca1e602161c)