tiny dead code

Author: pavel-kirienko

CLAUDE.md

@@ -67,6 +67,8 @@ TESTING ON UNSUPPORTED VERSIONS IS NOT ALLOWED.
 
 ACCEPTANCE CRITERIA: Work will not be accepted unless `nox` (without arguments) runs successfully.
 
+When starting work on a new feature, it is best to clean up temporary files using `nox -s clean`.
+
 ### Logging
 
 Logging is required throughout the codebase; prefer many short messages. Avoid adding logging statements on code

src/pycyphal/_header.py

@@ -4,7 +4,6 @@
 from dataclasses import dataclass
 
 U64_MASK = 0xFFFFFFFFFFFFFFFF
-U64_MODULO = 1 << 64
 
 HEADER_SIZE = 24
 SEQNO48_MASK = (1 << 48) - 1