cmd/go: remove go mod fix, add go help go.mod

"go mod fix" does work already done by nearly every other go command.
It was also confusing why we had both "go mod fix" and "go mod tidy".
Delete "go mod fix".

The main reason we kept "go mod fix" this long was for the discussion
of automatic go.mod updates in its documentation, which is now moved
into a new "go help go.mod".

Fixes #26831.

Change-Id: Ic95ca8918449ab79791d27998e02eb3377ac7972
Reviewed-on: https://go-review.googlesource.com/129682


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>