cmd/edit: stop double warning

[apainintheneck]

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When you don't have EDITOR or HOMEBREW_EDITOR configured brew edit printed a double warning.

This silences the first of those.

# First, unset EDITOR and HOMEBREW_EDITOR.

# Second, try it on master.
/u/l/Homebrew (master|✔) $ brew edit
Warning: Using code because no editor was set in the environment.
This may change in the future, so we recommend setting EDITOR,
or HOMEBREW_EDITOR to your preferred text editor.
Editing /usr/local/Homebrew
Warning: Using code because no editor was set in the environment.
This may change in the future, so we recommend setting EDITOR,
or HOMEBREW_EDITOR to your preferred text editor.

# Third, try it on this branch.
/u/l/Homebrew (stop-double-warning-in-edit|✔) $ brew edit
Editing /usr/local/Homebrew
Warning: Using code because no editor was set in the environment.
This may change in the future, so we recommend setting EDITOR,
or HOMEBREW_EDITOR to your preferred text editor.

[BrewTestBot]

Review period will end on 2023-02-14 at 00:00:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Thanks again @apainintheneck!