Remove unused type Copilot.Core.Expr.Tag. Refs #304.

The type Copilot.Core.Expr.Tag was deprecated in Copilot 3.6 because it
was not used and therefore did not belong in the program (see #260).

As per our internal policy of waiting 3 versions from deprecation until
a public interface declaration can be removed, this type can now be
removed completely.

The following demonstrates that the type is unused within Copilot:

```
$ grep -nHre '\<Tag\>' --include='*.hs' | grep -ve '^{-#' | grep -ve '--.*Tag' | grep -ve '"[^"]*Tag[^"]*"'
copilot-core/src/Copilot/Core/Expr.hs:15:  , Tag
copilot-core/src/Copilot/Core/Expr.hs:43:type Tag = Int
```

Only the declaration in the module Copilot.Core.Expr is shown.

This commit removes the declaration. Using the method above shows no
results.

Author: ivanperez-keera

copilot-core/src/Copilot/Core/Expr.hs

@@ -12,7 +12,6 @@ module Copilot.Core.Expr
   , Expr (..)
   , UExpr (..)
   , DropIdx
-  , Tag
   ) where
 
 import Copilot.Core.Operators (Op1, Op2, Op3)
@@ -38,12 +37,6 @@ type DropIdx = Word32
 
 --------------------------------------------------------------------------------
 
--- | A unique tag for external arrays/function calls.
-{-# DEPRECATED Tag "The type Tag is deprecated in Copilot 3.6." #-}
-type Tag = Int
-
---------------------------------------------------------------------------------
-
 -- | Internal representation of Copilot stream expressions.
 --
 -- The Core representation mimics the high-level Copilot stream, but the Core