Remove now-invalid assertion

This function can take one field from an object type, and another field
from an interface type--then their parent types are not equal.

crates/apollo-compiler/src/validation/selection.rs

@@ -318,8 +318,6 @@ pub(crate) fn fields_in_set_can_merge(
  field_a: FieldSelection<'_>,
  field_b: FieldSelection<'_>,
  ) -> Result<(), ValidationError> {
- debug_assert_eq!(field_a.parent_type, field_b.parent_type);
-
  // 2bi. fieldA and fieldB must have identical field names.
  if field_a.field.name != field_b.field.name {
  return Err(ValidationError::new(