[Autofill] Remove readonly, tabindex="-1" from test input files

This CL removes all readonly and tabinex="-1" attributes from the
form-parsing input files.

The reason is that
- the parameterized sectioning algorithm is sensitive to
  FormFieldData::is_focusable(), and
- on iOS, FormFieldData::is_focusable() requires that a field is
  non-readonly and its tabIndex is non-negative.

Therefore, this CL is a prerequisite for crrev.com/c/5079434.

Once the inconsistency between iOS and non-iOS form extraction has
been resolved, this CL can be reverted.

Bug: 40159136, 337179781
Change-Id: I74ea2d1115e7a7fbef1e5d524b5a98cfc6e733a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5514142
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1295983}