cmd/link, reflect: remove some method type data

cmd/link, reflect: remove some method type data

Remove reflect type information for unexported methods that do not
satisfy any interface in the program.

Ideally the unexported method would not appear in the method list at
all, but that is tricky because the slice is built by the compiler.

Reduces binary size:

	cmd/go: 81KB (0.8%)
	jujud: 258KB (0.4%)

For #6853.

Change-Id: I25ef8df6907e9ac03b18689d584ea46e7d773043
Reviewed-on: https://go-review.googlesource.com/21033
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>