internal/impl: remove Export.ExtensionDescFromType

This is no longer needed by the old implementation.

Change-Id: I3ba02d37f35f599ec790ec4e627258273883a308
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/223279
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/go.mod b/go.mod
index 0da18b0..89e2c13 100644
--- a/go.mod
+++ b/go.mod
@@ -3,6 +3,6 @@
 go 1.9
 
 require (
-	github.com/golang/protobuf v1.4.0-rc.4
+	github.com/golang/protobuf v1.4.0-rc.4.0.20200313231945-b860323f09d0
 	github.com/google/go-cmp v0.4.0
 )
diff --git a/go.sum b/go.sum
index 3b9b73d..1bbb8a5 100644
--- a/go.sum
+++ b/go.sum
@@ -1,8 +1,8 @@
 github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi5j83Wpe3EHw8=
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
 github.com/golang/protobuf v1.4.0-rc.2/go.mod h1:LlEzMj4AhA7rCAGe4KMBDvJI+AwstrUpVNzEA03Pprs=
-github.com/golang/protobuf v1.4.0-rc.4 h1:+EOh4OY6tjM6ZueeUKinl1f0U2820HzQOuf1iqMnsks=
-github.com/golang/protobuf v1.4.0-rc.4/go.mod h1:WU3c8KckQ9AFe+yFwt9sWVRKCVIyN9cPHBJSNnbL67w=
+github.com/golang/protobuf v1.4.0-rc.4.0.20200313231945-b860323f09d0 h1:aRz0NBceriICVtjhCgKkDvl+RudKu1CT6h0ZvUTrNfE=
+github.com/golang/protobuf v1.4.0-rc.4.0.20200313231945-b860323f09d0/go.mod h1:WU3c8KckQ9AFe+yFwt9sWVRKCVIyN9cPHBJSNnbL67w=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.4.0 h1:xsAVV57WRhGj6kEIi8ReJzQlHHqcBYCElAvkovg3B/4=
diff --git a/internal/impl/api_export.go b/internal/impl/api_export.go
index 76dfdf7..bde4754 100644
--- a/internal/impl/api_export.go
+++ b/internal/impl/api_export.go
@@ -168,12 +168,3 @@
 func (Export) MessageStringOf(m pref.ProtoMessage) string {
 	return prototext.MarshalOptions{Multiline: false}.Format(m)
 }
-
-// ExtensionDescFromType returns the legacy protoV1.ExtensionDesc for t.
-func (Export) ExtensionDescFromType(t pref.ExtensionType) *ExtensionInfo {
-	// TODO: Delete this function when v1 directly does this assertion.
-	if xt, ok := t.(*ExtensionInfo); ok {
-		return xt
-	}
-	return nil
-}