docs: remove unused release instruction (#8100)
docs: remove unused release instruction (#8100)

Turns out since we use `git archive`, there's no need for `git clean`
which was pretty disruptive.
apache/incubator-superset,airbnb/superset,apache/incubator-superset,airbnb/superset,airbnb/caravel,apache/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,mistercrunch/panoramix,airbnb/superset,zhouyao1994/incubator-superset,apache/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,airbnb/superset,zhouyao1994/incubator-superset,airbnb/caravel,airbnb/caravel,mistercrunch/panoramix,mistercrunch/panoramix,mistercrunch/panoramix