[FIX] web: fix tags field which looks weird on kanban view

For many2many_tags widget 'o_tag_color_0' class apply box shadow, in form
view it looks good but in kanban view, it does not look good so I have
removed box shadow from kanban view.

Before this commit in kanban view when no attributes are used in the field
at that time tags will be in box-shadow but after this commit even if no
attributes are used in the field, tags will be without box shadow.

Task ID: 1963188
Closes: #33503

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>