[slack] Remove ENABLE_GIST_PREVIEW config flag
[slack] Remove ENABLE_GIST_PREVIEW config flag

- SLACK.IS_ENABLED is enough for  new gist sharing
- We should not mix the old gist sharing with new

kawamon/hue,cloudera/hue,cloudera/hue,cloudera/hue,cloudera/hue,cloudera/hue,kawamon/hue,kawamon/hue,cloudera/hue,kawamon/hue,kawamon/hue,cloudera/hue,kawamon/hue,kawamon/hue,cloudera/hue,kawamon/hue,cloudera/hue,cloudera/hue,cloudera/hue,kawamon/hue,cloudera/hue,cloudera/hue,kawamon/hue,kawamon/hue,cloudera/hue,cloudera/hue,cloudera/hue,kawamon/hue,kawamon/hue,kawamon/hue,kawamon/hue,kawamon/hue,cloudera/hue,kawamon/hue,cloudera/hue,kawamon/hue