Drop custom version of search_env_filtered

It's now available in the utils cookbook, and having the two versions
create a conflict (with one overloading the other).
Merge pull request #330 from vuntz/drop-search

Drop custom version of search_env_filtered
Removed the single assignment block from constraints
Merge pull request #332 from tboerger/feature/roxy/fix-constraints

Removed the single assignment block from constraints