The -cpu flag is a weird one (and copied originally from Caddy), it
basically sets GOMAXPROCS which can be *easily* done by just setting
that environment variable. Also with systemd and containerized env you
set this externally *anyway*, so there is little use to do this again in
the binary.

Also the option's help was confusing (i.e. percentage of what?). Remove
the option and supporting files.
Signed-off-by: Miek Gieben <miek@miek.nl>