Since I cannot get to the text of the message, I cannot really accomplish this task. I can however Select All and Cut from the Edit menu using the following paths
Evolution → Compose Message → 1st <no name> → 1st <no name> → 1st <no name> → 1st <no name> → 1st <no name> → Edit → Select All → [Take Action]
Evolution → Compose Message → 1st <no name> → 1st <no name> → 1st <no name> → 1st <no name> → 1st <no name> → Edit → Cut → [Take Action]