remove(app-crypt/trousers): Drop unneeded TPM code.

This simplifies the build process, we have no need for trousers or other
tpm related things from ChromeOS. Bump vboot_reference so it no longer
needs trousers as a build dependency.
Merge pull request #362 from marineam/notss

remove(app-crypt/trousers): Drop unneeded TPM code.