Remove butane_core self-dev-dependency (#328)

Cargo publish now barfs on this, for details see
rust-lang/cargo#15151

The `async-adapter` specification was added in #281 but is no longer
necessary because it ends up transitively being included anyway
through the `butane_test_helper` dev dependency.

Author: Electron100

Cargo.lock

@@ -58,7 +58,6 @@ thiserror = "2.0"
 uuid = { workspace = true, optional = true }
 
 [dev-dependencies]
-butane_core = { workspace = true, features = ["log", "async-adapter"] }
 assert_matches = "1.5"
 butane_test_helper = { workspace = true, default-features = false, features = ["sqlite", "pg"] }
 butane_test_macros.workspace = true