database/sql: fix ctx.Done() == nil check

ctx.Done() == ctx.Background().Done() is just
a long way to write ctx.Done() == nil.
Use the short way.

Change-Id: I7b3198b5dc46b8b40086243aa61882bc8c268eac
Reviewed-on: https://go-review.googlesource.com/78128
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: rsc

src/database/sql/ctxutil.go

@@ -120,7 +120,7 @@ func ctxDriverBegin(ctx context.Context, opts *TxOptions, ci driver.Conn) (drive
 		}
 	}
 
-	if ctx.Done() == context.Background().Done() {
+	if ctx.Done() == nil {
 		return ci.Begin()
 	}