Remove dead code.

Fixes -Wunused-function warnings on gcc 4.7.
Remove unused goto targets.

Fixes -Wunused-label warnings on gcc 4.7.
Restore cue_is_incomplete pre caitp's review comments.

This is still unused, but helpful for debugging.
Merge pull request #287 from rillian/warning

More warning fixes.
Bug #853879 - Fix endless loop with malformed timestamp.
Additional work
Pass payload tests, fix leaks by releasing `webvtt_cue::body`

Payload tests were failing because `line_buffer` was not being
appended to `body` in `webvtt_finish_parsing()`. I've corrected this
by calling `read_cuetext()` before `webvtt_parse_cuetext()` in this
function.

Also now releasing `webvtt_cue::body` when cues are deleted.