[REF] web: remove useless oe_number rule
[REF] web: remove useless oe_number rule

This was introduced as a fix for grouped list views' headers while we
should have added the 'o_list_number' class instead... which is the
case now. So the oe_number rule became useless.

See https://github.com/odoo/enterprise/commit/a2ce226a2a085b26174cdfd32922517dff39658c
See https://github.com/odoo/odoo/commit/5d13a2f431e1c96e075b04d0eb07d98b912a6547
See https://github.com/odoo/odoo/pull/23966

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