[release-branch.go1] pkg: Removing duplicated words ("of of", etc.), mostly from...

[release-branch.go1] pkg: Removing duplicated words ("of of", etc.), mostly from comments.

««« backport ce3a981ad1ef
pkg: Removing duplicated words ("of of", etc.), mostly from comments.

Ran 'double.pl' on the pkg tree to identify doubled words.
One change to an error string return in x509;  the rest are in comments.
Thanks to Matt Jibson for the idea.

R=golang-dev, bsiegert
CC=golang-dev
https://golang.org/cl/6344089

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