Remove sahara-dashboard

sahara-dashboard has been removed in the commit:
  Change-Id: I1f0e93e1dee3d065c4f00d8bf2042bebc8d45a22
However, there is still sahara-dashboard in extras.d/70-sahara.sh. This
causes an error.

/opt/stack/new/devstack/extras.d/70-sahara.sh: line 7:
/opt/stack/new/devstack/lib/sahara-dashboard: No such file or directory

So this commit just removes it.

Change-Id: Iae76dc5f92a1f0a557d43d74b9b5d2c347cc23a9
Closes-Bug: #1363828
Merge "Remove sahara-dashboard"