[cfe] Remove obsolete nnbd_api.md

This document was created during the early stages of the NNBD
development. The goal was to aid the migration to the new model. The
document has served its purpose and is not maintained for some time
now, so it can be deleted.

Change-Id: I398331cc41a0070e28cf3d1f94da958dd2d3ecfa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436602
Commit-Queue: Chloe Stefantsova <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: chloestefantsova