Revert outline summarization (#3114)

This pull request essentially reverts #3067: we noticed that only using
the function signatures produces far worse results in codegen, and so
that feels like a regression compared to before. We should re-enable
this once we have a smarter approach to fetching context during codegen,
possibly when #3097 lands.

As a drive-by, we also fixed a longstanding bug that caused codegen to
include the final line of a selection even if the selection ended at the
start of the line.

Ideally, I'd like to hot fix this to preview so that it goes to stable
during the weekly release.

/cc: @KCaverly @nathansobo 

Release Notes:

- N/A