[release-branch.go1] doc/talks: remove talks, refer to talks.golang.org instead

««« backport e3b986984e53
doc/talks: remove talks, refer to talks.golang.org instead

These talks have been moved to the go.talks sub-repository:
        https://code.google.com/p/go.talks

R=rsc, r
CC=gobot, golang-dev
https://golang.org/cl/6529052

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