Revert "Install Linux 4.7."

This reverts commits:

* ab562fd4
* c0bc655c
* 80d0667a

The last minute attempt to include it in Tails 2.6 was a bad (too hasty)
idea, e.g. USB installations don't boot (pre-init results in
"FAT-fs (sdb1): IO charset ascii not found"). Also it seems that the
backport of Linux 4.7 hasn't gone through the proper Debian QA process.