math/big: remove "else" from if with block that ends with return

That "else" was needed due to gc DCE limitations.
Now it's not the case and we can avoid go lint complaints.
(See #23521 and https://golang.org/cl/91056.)

There is inlining test for bigEndianWord, so if test
is passing, no performance regression should occur.

Change-Id: Id84d63f361e5e51a52293904ff042966c83c16e9
Reviewed-on: https://go-review.googlesource.com/104555Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/math/big/nat.go

@@ -1245,9 +1245,8 @@ func (z nat) bytes(buf []byte) (i int) {
 func bigEndianWord(buf []byte) Word {
 	if _W == 64 {
 		return Word(binary.BigEndian.Uint64(buf))
-	} else { // Explicit else is required to get inlining. See #23521
-		return Word(binary.BigEndian.Uint32(buf))
 	}
+	return Word(binary.BigEndian.Uint32(buf))
 }
 
 // setBytes interprets buf as the bytes of a big-endian unsigned