Remove scripts/release.sh because of no longer used (#1137)

scripts/release.sh is used for release plugins manually (by
maintainer's hand), previously. Now we introduced automated release
process by github action, hence it is no longer used and no longer
maintained. This change removes this file. Thanks, release.sh for a
long time!

Signed-off-by: Tomofumi Hayashi <tohayash@redhat.com>

Author: s1061123