Revert "Throw an error if androidboot.hardware is used for bootconfig"

This reverts commit 2e43e34d.

The kernel bootconfig feature has been updated to handle mixed subkeys
and values, so androidboot.hardware is ok to use now.

Test: build and boot cuttlefish with 'hardware=cutf_vm'
Test: build and boot cuttlefish with 'androidboot.hardware=cutf_vm'
Bug: 191502832
Merged-In: I7c3f330a73abe6cf1fb7d14aaaa6cbc3e874a4aa
Change-Id: I7c3f330a73abe6cf1fb7d14aaaa6cbc3e874a4aa