collab: Remove unused parameter to run_database_migrations (#15883)

This PR removes the unused `ignore_checksum_mismatch` parameter to
`run_database_migrations`.

We were always passing `false`, which meant the behavior didn't need to
be parameterized.

Release Notes:

- N/A

crates/collab/src/llm/db/tests.rs

@@ -81,7 +81,7 @@ impl TestLlmDb {
                 .await
                 .unwrap();
             let migrations_path = concat!(env!("CARGO_MANIFEST_DIR"), "/migrations_llm");
-            run_database_migrations(db.options(), migrations_path, false)
+            run_database_migrations(db.options(), migrations_path)
                 .await
                 .unwrap();
             db