[vm/tfa] Remove assertion in the TFA.

This assertion turns out to be incorrect, see
tests/language/class/class_test.dart for an example.

TEST=No behavior change

Change-Id: I4a9454364f3d36f67df7fb6d803ac7bfd62fbbd2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251681
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Joshua Litt <joshualitt@google.com>

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -187,8 +187,6 @@ class MoveFieldInitializers {
         if (!_isRedirectingConstructor(c)) c
     ];
 
-    assert(constructors.isNotEmpty || cls.isMixinDeclaration);
-
     // Move field initializers to constructors.
     // Clone AST for all constructors except the first.
     bool isFirst = true;