Partially revert "Update CHANGELOG for Firestore v0.14.0 (#2025)" (#2031)

This removes the changelog entry that describes our migration to gRPC-C++.

Author: wilhuff

Firestore/CHANGELOG.md

@@ -2,12 +2,6 @@
 
 # v0.14.0
 -[fixed] Fixed compilation in C99 and C++11 modes without GNU extensions.
--[changed] Moved from Objective-C gRPC framework to gRPC C++. If you're
- manually tracking dependencies, the `gRPC`, `gRPC-ProtoRPC`, and
-`gRPC-RxLibrary` frameworks have been replaced with `gRPC-C++`. While we
- don't anticipate any issues, please [report any issues with network
- behavior](https://github.com/firebase/firebase-ios-sdk/issues/new) you
- experience. (#1968)
 
 # v0.13.6
 -[changed] Internal improvements.