Correct information on dylib compression

Compression of dylibs was removed in rust-lang#113695 (and decompression removed in rust-lang#132402).

Author: ehuss

src/doc/rustc-dev-guide/src/backend/libs-and-metadata.md

@@ -42,7 +42,7 @@ format is specific to `rustc`, and may change over time. This file contains:
 ### dylib
 
 A `dylib` is a platform-specific shared library. It includes the `rustc`
-[metadata] in a special link section called `.rustc` in a compressed format.
+[metadata] in a special link section called `.rustc`.
 
 ### rmeta