gofmt: remove redundant check in rewriter
gofmt: remove redundant check in rewriter

If the actual types of two reflect values are
the same and the values are structs, they must
have the same number of fields.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/108280043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go