[Autofill] Eliminate AttributeInstance::GetTopLevelType()

All existing calls of GetTopLevelType() are passed to
GetNormalizedType(), which
- if the argument type is supported:
  returns the argument type, which happens to be GetTopLevelType().
- if the argument type is the AttributeType::field_type():
  returns GetTopLevelType().

That's equivalent to just passing AttributeType::field_type() instead
of GetTopLevelType().

This CL does that and removes GetTopLevelType().

Bug: 389884340, 398809224
Change-Id: Ie081656650fdcdd9fc95e28a50c2484d05304b04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6331547
Reviewed-by: Jan Keitel <jkeitel@google.com>
Auto-Submit: Christoph Schwering <schwering@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1428832}