[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#82486

X-original-commit: b3b9627b655cd7cb928925affed6cc8d92661e8d
Related: odoo/enterprise#23364
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

setup/package.dfdebian

@@ -28,7 +28,6 @@ RUN apt-get update -qq &&  \
         python3-decorator \
         python3-docutils \
         python3-gevent \
-        python3-html2text \
         python3-pil \
         python3-jinja2 \
         python3-libsass \

setup/package.dffedora

@@ -21,7 +21,6 @@ RUN dnf update -d 0 -e 0 -y && \
         python3-freezegun \
         python3-gevent \
         python3-greenlet \
-        python3-html2text \
         python3-idna \
         python3-jinja2 \
         python3-lxml \