Python: Only one Unit implementation

Conflict arose since the Unit in DataFlowPrivate was added in a merged PR.

The behavior from this PR will make it match what java does (931322e4c5/java/ql/src/semmle/code/Unit.qll)

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1151,14 +1151,5 @@ predicate isImmutableOrUnobservable(Node n) { none() }
 
 int accessPathLimit() { result = 5 }
 
-/** The unit type. */
-private newtype TUnit = TMkUnit()
-
-/** The trivial type with a single element. */
-class Unit extends TUnit {
-  /** Gets a textual representation of this element. */
-  string toString() { result = "unit" }
-}
-
 /** Holds if `n` should be hidden from path explanations. */
 predicate nodeIsHidden(Node n) { none() }