iommu_gas: drop fini cleanup loop

Assertions suggest that the loop in iommu_gas_fini_domain is executed
zero times, so remove it.

Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D37204

(cherry picked from commit a2c57c60d9e2667744357fdadd79a98ce64d3ad9)