The support for slow collations is documented as deprecated and "will be removed in 5.0".
Here is a patch. It removes:
Commit 1661720 from Adrien Grand in branch 'dev/trunk'
[ https://svn.apache.org/r1661720 ]
LUCENE-6281: Removed slow collation support.
Commit 1661721 from Adrien Grand in branch 'dev/branches/branch_5x'
[ https://svn.apache.org/r1661721 ]
Bulk close after 5.1 release