Remove in-rust-tree feature

Since it's unused for now -it'll be re-introduced along with the
upcoming `proc-macro-srv/sysroot` feature.

Author: fasterthanlime

crates/rust-analyzer/Cargo.toml

@@ -84,4 +84,3 @@ mbe = { path = "../mbe" }
 [features]
 jemalloc = ["jemallocator", "profile/jemalloc"]
 force-always-assert = ["always-assert/force"]
-in-rust-tree = []