internal/impl: remove unused ExtensionField.SetLazy method

This cleanup was done in June 2020 by Dan Scales,
but never upstreamed to the open source Go Protobuf repository.

Change-Id: I175d9588b5bb536fa276133f8fe6073278e6d3b0
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/605895
Reviewed-by: Christian Höppner <hoeppi@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/impl/codec_extension.go

@@ -67,7 +67,6 @@ type lazyExtensionValue struct {
 	xi         *extensionFieldInfo
 	value      protoreflect.Value
 	b          []byte
-	fn         func() protoreflect.Value
 }
 
 type ExtensionField struct {
@@ -158,10 +157,9 @@ func (f *ExtensionField) lazyInit() {
 		}
 		f.lazy.value = val
 	} else {
-		f.lazy.value = f.lazy.fn()
+		panic("No support for lazy fns for ExtensionField")
 	}
 	f.lazy.xi = nil
-	f.lazy.fn = nil
 	f.lazy.b = nil
 	atomic.StoreUint32(&f.lazy.atomicOnce, 1)
 }
@@ -174,13 +172,6 @@ func (f *ExtensionField) Set(t protoreflect.ExtensionType, v protoreflect.Value)
 	f.lazy = nil
 }
 
-// SetLazy sets the type and a value that is to be lazily evaluated upon first use.
-// This must not be called concurrently.
-func (f *ExtensionField) SetLazy(t protoreflect.ExtensionType, fn func() protoreflect.Value) {
-	f.typ = t
-	f.lazy = &lazyExtensionValue{fn: fn}
-}
-
 // Value returns the value of the extension field.
 // This may be called concurrently.
 func (f *ExtensionField) Value() protoreflect.Value {