Don't use lexer to read timestamps.

The goal is to simplify and eventually remove the lexer.

webvtt_collect_timings_and_settings no longer relies on the lexer
for parsing integers, percentages, or timestamps, nor any of the
cuesetting keywords/special values. Therefore a significant chunk
of the lexer is now deletable. Horray.
Remove unused code from lexer.

Removes cue-setting keywords, "NOTE" keyword, integers/percentages, timestamps,
and operators like "-->" and "." from the lexer code, as these are not used in
the code.