Remove unnecessary 'pub' from test module, and remove dead constant.

After making the test module private, the compiler noticed and warned
that the constant is unused.

pageserver/src/layered_repository.rs

@@ -1085,7 +1085,7 @@ pub mod repo_harness {
 }
 
 #[cfg(test)]
-pub mod tests {
+mod tests {
     use super::metadata::METADATA_FILE_NAME;
     use super::*;
     use crate::keyspace::KeySpaceAccum;
@@ -1467,12 +1467,6 @@ pub mod tests {
         Ok(())
     }
 
-    // Target file size in the unit tests. In production, the target
-    // file size is much larger, maybe 1 GB. But a small size makes it
-    // much faster to exercise all the logic for creating the files,
-    // garbage collection, compaction etc.
-    pub const TEST_FILE_SIZE: u64 = 4 * 1024 * 1024;
-
     #[test]
     fn test_images() -> Result<()> {
         let repo = RepoHarness::create("test_images")?.load();