Remove the -Zsave-analysis indexer

Summary:
The flag itself no longer exists in rustc and this indexer never quite worked for our use cases.
We will use rust-analyzer instead, which has SCIP and LSIF support already.

-  D45292993 removed the build flavor we relied on
- see  rust-lang/rust#43606 for removal

Reviewed By: simonmar

Differential Revision: D45516746

fbshipit-source-id: 684117415c5daebca7656c330592f1362781ab1c

Author: donsbot