fix: remove functions definitions w/ invalid return types

T? and T|? are not valid return types

T|? is equivalent to setting the nullability to MIRROR
T&? is not currently expressible via nullability

BREAKING CHANGE: least_skip_null and greatest_skip_null functions have been removed

extensions/functions_comparison.yaml

@@ -225,18 +225,8 @@ scalar_functions:
           - value: T
         variadic:
           min: 2
-        return: T|?
-  -
-    name: "least_skip_null"
-    description: >-
-      Evaluates each argument and returns the smallest one. The function will return
-      null only if all arguments evaluate to null.
-    impls:
-      - args:
-          - value: T
-        variadic:
-          min: 2
-        return: T&?
+        return: T
+        nullability: MIRROR
   -
     name: "greatest"
     description: >-
@@ -247,15 +237,5 @@ scalar_functions:
           - value: T
         variadic:
           min: 2
-        return: T|?
-  -
-    name: "greatest_skip_null"
-    description: >-
-      Evaluates each argument and returns the largest one. The function will return
-      null only if all arguments evaluate to null.
-    impls:
-      - args:
-          - value: T
-        variadic:
-          min: 2
-        return: T&?
+        return: T
+        nullability: MIRROR