all: remove some manual hyphenation

Manual hyphenation doesn't work well when text gets reflown,
for example by godoc.

There are a few other manual hyphenations in the tree,
but they are in local comments or comments for unexported functions.

Change-Id: I17c9b1fee1def650da48903b3aae2fa1e1119a65
Reviewed-on: https://go-review.googlesource.com/53510


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>