pyproject: drop sigstore-protobuf-specs dependency

Author: woodruffw

pyproject.toml

@@ -37,8 +37,6 @@ dependencies = [
   "rich >= 13,< 15",
   "rfc8785 ~= 0.1.2",
   "rfc3161-client >= 1.0.3,< 1.1.0",
-  # NOTE(ww): Both under active development, so strictly pinned.
-  "sigstore-protobuf-specs == 0.5.0",
   "sigstore-rekor-types == 0.0.18",
   "tuf ~= 6.0",
   "platformdirs ~= 4.2",