h/tws.h: Remove TW_YES and TW_NO; TW_SUCC good enough.

h/tws.h: Remove TW_YES and TW_NO;  TW_SUCC good enough.

TW_SUCC is a macro for the bit-mask 0x0040.  TW_YES, a vague name, was
the same value.  TW_NO was 0, and unused by the code.  Ditch both those
as TW_SUCC is sufficient for setting and testing.  Alter its comment to
make clear it means parsing was successful.