Simplify FormStructureRationalizer::RationalizeTypeRelationships()

The function is implemented as a somewhat generic mechanism to
rationalise certain types to UNKNOWN_TYPE if certain other types are not
present. In reality, it is only used for phone country codes. This CL
makes the code less generic and is explicit about the fact that this is
only relevant for PHONE_HOME_COUNTRY_CODE.

The generic mechanism seems unnecessary, given that
form_structure_rationalization_engine exists.

Bug: 40100455
Change-Id: I46369b82a0853e2c9a96b97d226fb713e4c0d672
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6003002
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1379614}