Simplify the RELEASING steps based on current release.sh.

Since release.sh creates and pushes a libdrm-$VERSION tag for us,
there's no need to also have the user manually generating a $VERSION
tag as well.

I also dropped the "optional" part of distcheck.  You shouldn't have
pushed master with a version bump that hasn't passed distcheck.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>