Make mouse-wheel-scroll-amount consistent among platforms

* lisp/term/ns-win.el (mouse-wheel-scroll-amount): Do not put any
values on mouse-wheel-scroll-amount on ns.
1 job for master in 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #2036


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 sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 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)