Revert "[FIX] web: Can't save a x2m in list editable if contains a m2m_tags with a color field"

Reverted because it causes issue #10083 when serializing
nexted x2many values, and produces invalid x2many commands,
such as:

    `[[6, false, [[5, false, false], [6, false, [8]]]]]`

Pending a proper fix for the oririginal issue.

This reverts commit 7e2628ae.

Closes #10083