Remove deleted pipeline from README (#934)

Author: hlopko

README.md

@@ -1,7 +1,6 @@
 # Rust Rules
 
 * Postsubmit [![Build status](https://badge.buildkite.com/76523cc666caab9ca91c2a08d9ac8f84af28cb25a92f387293.svg?branch=main)](https://buildkite.com/bazel/rustlang-rules-rust-postsubmit?branch=main)
-* Postsubmit + Current Bazel Incompatible Flags [![Build status](https://badge.buildkite.com/64e1f18bbd4cb612f8efd44f755f8dc493ffd69b7140d973a9.svg?branch=main)](https://buildkite.com/bazel/rules-rust-plus-bazelisk-migrate)
 
 ## Overview