Cleanup a few things in emulators/qemu-cheri. No functional change.

• Remove unused pkg-plist and PLIST=/dev/null setting to ignore it.
• Remove unused QEMU_SYSTEM_CHERI variable.
• Synchronize USES with emulators/qemu a bit.

Reviewed by: brooks (maintainer)
Approved by: brooks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25529