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.
Fix warnings in `webvtt_cue_validate_set_settings`

Uninitialized variable warning fixed
Merge pull request #413 from caitp/issue-413

Remove unnecessary code from lexer (DEPENDS ON #414)