Revert "[attributes] Add a facility for enforcing a Trusted Computing Base."

This reverts commit c163aae45ef6b7f3bd99601195d3ce4aad5850c6.
Doesn't compile on some bots
(http://lab.llvm.org:8011/#/builders/98/builds/3387/steps/9/logs/stdio),
breaks tests on bots where it does compile
(http://45.33.8.238/linux/36843/step_7.txt).