Fix compilation warning in qp.el

* lisp/mail/qp.el (quoted-printable-encode-region): No need to
convert the regexp to multibyte before searching.
1 job for master in 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #2133


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:e1de74e67cc70422153600785c4ce4c29843a0e59e18b634c52ca133c564437f for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)