The `accounts.email.accounts.<name>.neomutt.extraConfig` option is
included twice in the resulting config file for the account. One time as
part of the `mraSection`, one time as part of `accountStr` (`accountStr`
includes the `mraSection`). This removes that duplication. I opted to
keep the one in `accounStr`, since `extraConfig` doesn't necessarily
have anything to do with the `mraSection`.
This resolves the error
The option `accounts.email.accounts.xyz.neomutt.sendMailCommand`
is defined both null and not null, in
`…/home-manager/modules/accounts/email.nix' and
`…/home-manager/modules/accounts/email.nix'.
that would occur previously when both neomutt and msmtp were enabled
for an account.
