$RLWRAP_FILTERDIR should be removed from $PATH by RlwrapFilter.{pm,py}

[hanslub42]

When writing a filter php that calls php (the PHP interpreter) to harvest function names etc. for completion, chaos ensues as the filter calls itself, not the interpreter.

I cannot think of a situation where a filter wants to call another filter, so it should be safe to remove $RLWRAP_FILTERDIR from $PATH within RlwrapFilter.pm