[FIX] l10n_it: fix tax deductible percent and remove obsolete taxes

tax name suggest 10 but code 20, history tells 10 is right, fixed code
two children taxes are now orphans, kill the orphans

closes odoo/odoo#40825

X-original-commit: 28e74fafec4cec3179db330a1ededf4ef8677fbc
Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>