Removed invalid options for the sendmail transport

[kylefarris]

Removed invalid options for the sendmail transport that were causing Nodemailer to return the SMTPTransport instead.

Fixes #8575

[CLAassistant]

All committers have signed the CLA.

[pitaj]

I think these should be moved to apply to the SMTP transport instead, not removed entirely.

Also, just removing them from the code doesn't remove the options from the admin menu.

[kylefarris]

Before we add those options to the SMTP transport, we should consider that Nodemailer has deprecated the options and will be removing them in future updates: https://nodemailer.com/smtp/pooled/#deprecated-options

I can still make the swap if you'd like, but, we could also just remove the options from the interface entirely. At the very least, a note should be made in the code about the deprecation.

[pitaj]

Ah, in that case, remove rateLimit and rateDelta entirely, and add pool to the SMTP options. Maybe even make it an nconf option.

Regardless, you'll need to make changes to the admin templates and translations to remove the sendmail options.

[kylefarris]

No problem. I removed the deprecated options from the admin interface and added a new one for toggling pooled connections for the SMTP Transport. Tested on my dev machine and it all looks good there. Tests passed on there as well.

[barisusakli]

LGTM