impl: remove unnecessary nil check from presence.Present

The presence type is always constructed with a non-nil pointer,
so this check can never trigger.

I was not able to measure a performance difference either way.
(See Google-internal CL 759581451 for test results.)

Change-Id: I73c43fbca91ef59c3f96b43f4687e68813e58fa2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/674015
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

internal/impl/presence.go

diff --git a/internal/impl/presence.go b/internal/impl/presence.go
index 914cb1d..443afe8 100644
--- a/internal/impl/presence.go
+++ b/internal/impl/presence.go
@@ -32,9 +32,6 @@
 
 // Present checks for the presence of a specific field number in a presence set.
 func (p presence) Present(num uint32) bool {
-	if p.P == nil {
-		return false
-	}
 	return Export{}.Present(p.toElem(num), num)
 }