Clean up unused files and update README in Fireperf. (#2365)

* Clean up unused files.

* Update README.md

Author: jeremyjiang-dev

firebase-perf/README.md

@@ -7,7 +7,7 @@ For more information about app performance and many other cool mobile services,
 
 ## Clone the repository
 
-Follow the instruction in the [Root README](https://github.com/firebase/firebase-android-sdk/blob/master/README.md)
+Follow the instruction in the [Root README](https://github.com/firebase/firebase-android-sdk#getting-started)
 to clone the repository.
 
 ## Local Environment Setup