[REF] various: remove usage and dependency on html2text library

We have our own html2plaintext, already used in lot of use cases instead of
just a few for the html2txt library.

Notably for emails: most emails going through Odoo stack use our simple
html2plaintext to format the body alternative. When no body alternative
is given to ``build_email`` an alternative is built using the library to
remove. Using our own parser allows to have the same results compared to
using ``MailMail.send()``. Difference lies in spaces and new lines as well
as markdown. Our html2plaintext is a bit simple and does not try to generate
Markdown but generates a simple plaintext version.

This also helps solving some issues with depending on that library.

Task-2702034

closes odoo/odoo#82330

X-original-commit: odoo/odoo@9b5f86e8fa5bef341ea7392b86987092d6f6933a
Related: odoo/enterprise#23292
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>