cmd/gofmt: remove -tabwidth and -tabs flags

cmd/gofmt: remove -tabwidth and -tabs flags

Having these flags misleads people into thinking they're acceptable
for code that "must be gofmt'd".

If an organization wishes to use gofmt internally with
different settings, they can fork gofmt trivially. But "gofmt"
as used by the community with open source Go code should not
support these old knobs.

Also removes the -comments flag.

Fixes #7101

R=r, gri
CC=golang-codereviews
https://golang.org/cl/52170043