proto: revert expose package-private equalField
proto: revert expose package-private equalField

This reverts commit 4d6d2135eba33eaef1c020129188ce7e82aa4270.

Post-submit discussion on CL 448876 suggests that this API should
change; revert it pending a decision.

Change-Id: I2e81be847c62ea7de5dfef36bb10ab193132e612
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/450955
Reviewed-by: Michael Stapelberg <97e0203ad32d728c266b6422d4bd97c18b9b600f@google.com>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Lasse Folger <c1a9282e9a97d9bcfa578c9e6643dbf90dcebfeb@google.com>
Run-TryBot: Lasse Folger <c1a9282e9a97d9bcfa578c9e6643dbf90dcebfeb@google.com>
TryBot-Bypass: Michael Stapelberg <97e0203ad32d728c266b6422d4bd97c18b9b600f@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go