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 <stapelberg@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Lasse Folger <lassefolger@google.com>
Run-TryBot: Lasse Folger <lassefolger@google.com>
TryBot-Bypass: Michael Stapelberg <stapelberg@google.com>
diff --git a/proto/equal.go b/proto/equal.go
index 12d064d..67948dd 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -74,11 +74,6 @@
 	return equalUnknown(mx.GetUnknown(), my.GetUnknown())
 }
 
-// EqualField compares two fields.
-func EqualField(fd protoreflect.FieldDescriptor, x, y protoreflect.Value) bool {
-	return equalField(fd, x, y)
-}
-
 // equalField compares two fields.
 func equalField(fd protoreflect.FieldDescriptor, x, y protoreflect.Value) bool {
 	switch {