Always use system ycpc

The README.md suggests to install yast2-core from our testing repo,
which will bring our custom ycpc binary into the system. There should be
no reason not to use it.

If someone really really needs to use a non-system ycpc, he can set up a
symlink.
Merge pull request #354 from yast/always-use-system-ycpc

Always use system ycpc