Remove support for Parser.filter

This was already only available for parsers that have no AST
transformations and don't rely on inlining. My last optimizations to the
token lists on the stack have broken this feature (because the new lists
are no longer mutable), but we don't use it anyway, so just remove it.

Even the original SableCC thesis says "We do not recommend this usage
[the Parser.filter method], because it is error prone."