CL 21462 and CL 21463 made this message say explicitly that the problem
was a struct field in a map, but the word "directly" is unnecessary,
sounds wrong, and makes the error long.

Change-Id: I2fb68cdaeb8bd94776b8022cf3eae751919ccf6f
Reviewed-on: https://go-review.googlesource.com/23373
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>