[FIX] base: return all the css attachments

When detecting if the assets where already preprocessed, the method
only the last asset type matching was returned and deleted.
In standard, it should not have any impact as all assets are of the
same type but it may be possible to mix it with external modules.

Sign CLA

closes odoo/odoo#79458

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>