Remove in-memory locks from distribution database (#11412)

## Summary

I believe these are not necessary... They're currently used in two
places:

1. When building wheels. But that's already wrapped in an in-flight map,
which does the same thing.
2. When fetching source distribution metadata. But every route there
uses it's own `flock` to coordinate across processes, so this seems
redundant?

crates/uv-distribution/src/lib.rs

@@ -14,7 +14,6 @@ mod distribution_database;
 mod download;
 mod error;
 mod index;
-mod locks;
 mod metadata;
 mod reporter;
 mod source;