wendelin: Unpin wendelin.core from versions in -dev variant

This is required so that buildout does not fallback to installing non-dev egg if version of wendelin.core from -dev differs from what has been pinned.

For example the following

[buildout]
extends =
  https://lab.node.vifib.com/nexedi/slapos/raw/1.0.12/software/wendelin/software-dev.cfg

# pin wendelin.core-dev to latest assumed-good revision with ZBlk1 support
[wendelin.core-repository]
revision = c507d9009f59fec2041bac9c31c5b08a48d3897d

will install wendelin.core-0.4.egg from pypi instead of installing c507d9009f59fec2041bac9c31c5b08a48d3897d from repository, because that latter revision says it is already version 0.5 and 1.0.12 wendelin SR pins wendelin.core to 0.4 .

So unpin wendelin.core from versions and let software-dev.cfg work always.

/cc @Tyagov, @klaus