[IMP] web: force desktop mode on mobile

Before this commit, some views appeared broken on mobile.
We don't consider this as a bug because Odoo is not supposed
to be responsive in Community edition. This feature is only
available on Enterprise.

This commit aims at clarifying this situation by clearly moving
the base requirements to support browsers' responsive to the
Enterprise edition and stating that the Community edition only
targets desktop.

It's important to note that, for technical reasons, some
responsive-related JS/CSS code may still be present in the
Community codebase but they are not meant to provide any
explicit or implicit support for responsive/mobile devices.

Technically speaking, this commit removes the viewport meta tag.
This change will then force the "desktop-like" rendering even on
small screens for the Community edition and will keep the current
behavior (scaling/zooming on small screens) for the Enterprise edition.

By doing this we do not let the users think that something is broken
and must be fixed.

So,
in Community edition, desktop is always displayed;
In Enterprise edition, Responsive Web Design is available.

We also had to adapt a test to be able to apply XPath adding
viewport meta tag in enterprise.

Task ID: 2090202

closes odoo/odoo#38768

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>