Revert "api_core: Make PageIterator.item_to_value public. (#4702)"

[theacodes]

This reverts commit 813a27e.

This is done to prevent a release-the-world scenario in order to release bigquery, spanner, and datastore. It will be re-reverted before the next minor release of api_core.

[dhermes]

@jonparrott This is just a straight git revert. You also need to update spanner/google/cloud/spanner_v1/client.py and spanner/google/cloud/spanner_v1/instance.py:

$ git log -1 --pretty=%H
d2ff0b9a68b55dff09b905736df7ddaaf722ce95
$ git grep -l '\.item_to_value'
api_core/google/api_core/page_iterator.py
api_core/tests/unit/test_page_iterator.py
bigquery/tests/unit/test_table.py
datastore/google/cloud/datastore/query.py
spanner/google/cloud/spanner_v1/client.py
spanner/google/cloud/spanner_v1/instance.py

[theacodes]

@dhermes good call.

[dhermes]

LGTM