Remove the PERLIO * argument to S_filter_gets(), as it's always PL_rsfp

Remove the PERLIO * argument to S_filter_gets(), as it's always PL_rsfp

Conceptually it's also wrong, as if there are source filters, the passed-in
file handle is not passed up the stack of filters for the topmost filter to
use to read from. It was in the parameter list from the first creation of
filter_gets() in 16d20bd98cd29be76029ebf04027a7edd34d817b, when calls to
sv_gets() were replaced by it.