Remove the `request_nogo` setting (#4761)

**What type of PR is this?**

Other

**What does this PR do? Why is it needed?**

`//go/private:request_nogo` was added in #2922 so that tools built in
the legacy host configuration, which couldn't apply the transition
`go_context_data` used to set it to `True`, would fall back to the noop
nogo. Host mode is gone, #4611 removed that transition and flipped the
default to `True`, and since #4755 no rules_go transition sets it at
all. `nogo_active` therefore only ever depends on `bootstrap_nogo`, the
actual cycle guard that `go_tool_transition` sets for nogo and other Go
tools.

This removes the setting and leaves `bootstrap_nogo` as the only nogo
setting:

* `nogo_active` only checks `bootstrap_nogo`.
* `bootstrap_nogo = False` is part of the dict of default values that
the tool transitions start from, and only `go_tool_transition` sets it
to `True`. The separate dict for the tool value and the overrides in the
non-Go tool and stdlib transitions that reset it to `False` are gone.

**Which issues(s) does this PR fix?**

**Other notes for review**