(regex) Minor Pattern cleanup

When building Pattern with character classes, e.g., "[a-z]", we redundantly create two different - but identical - BmpCharPredicate lambdas; we could instead turn the simple BitClass class into a BmpCharPredicate instead (it currently extends from CharProperty/Node, but is no longer used as such). Fixing this reduces allocations when compiling a common type of regexes.

This inefficiency is technically a regression since JDK-6328855 et al, but the impact is small.

There are also a few instances of other smells, like unused variables/classes etc, that should be cleaned up.