Remove expression duration

The Duration expression is unused and adds unnecessary complexity to the expression system. Removes the Duration class, its DSL method, and associated tests.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: jnunemaker

lib/flipper.rb

@@ -108,10 +108,6 @@ def time(value)
     Expression.build({ Time: value })
   end
 
-  def duration(scalar, unit = 'second')
-    Expression.build({ Duration: [scalar, unit] })
-  end
-
   def feature_enabled(name)
     Expression.build({ FeatureEnabled: name })
   end