Removes spurious equals sign from REFLECTMETHOD macro.

Fixes #29487

Change-Id: Iaa3d85ff57087aa79a259f28816f8b0a552536f3
Reviewed-on: https://go-review.googlesource.com/c/155927
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>