Remove FormDataAndroid::GetFieldIndex()

This function is around 9 years old [1] and predates the introduction of
`FieldGlobalId`. Instead it matches fields by looking at the equality
comparator IdenticalAndEquivalentDomElements().

ID checks are way cheaper, and are justified in the only two references
of the function. Therefore this CL removes it.

[1] https://codereview.chromium.org/2839023003

Bug: 456526604
Change-Id: Ib913724b257e8522939535f9016b4c9c88024ac3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7595305
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Friedrich Hauser <friedrichh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1589272}