misc: delete editor and shell support
We are not the right people to support editor plugins, and the profusion
of editors in this CL demonstrates the unreality of pretending to do so.
People are free to create and advertise their own repos with support.
For discussion: https://groups.google.com/forum/#!topic/golang-dev/SA7fD470FxU
LGTM=rminnich, kamil.kisiel, gri, rsc, dave, josharian, ruiu
R=golang-codereviews, rminnich, kamil.kisiel, gri, rsc, dominik.honnef, dave, josharian, ruiu, ajstarks
CC=golang-codereviews
https://golang.org/cl/
105470043