Cleanup from OPTE external IP workaround

[bnaecker]

This is a tracking issue for reverting the changes required to integrate with oxidecomputer/opte#167, which implements a workaround that co-opts instance source NAT to allow inbound connectivity.

• Do not set secondary MACs on underlay VNICs #1335
• Do not add default OPTE route to internet gateway #1336
• Don't muck with xde.conf #1337