mail/mu4e: Remove port
mu4e has become integrated as default to its backend port mail/mu. So
point users of mail/mu4e to mail/mu.
PR: 289593
Reported by: RwN <rwn@mailo.com>
Tested by: Trenton Schulz <trueos@norwegianrockcat.com>
Approved by: hrs (maintainer, timeout >5 weeks)