Remove non-existent trusty image references (#46594)
Summary:
Simplifies some parts of build.sh and removes old references in the code to non-existent trusty images.
There are other parts of the code where trusty is referenced for travis (most of them in third party directories) and I did not touch those. https://github.com/pytorch/pytorch/search?q=trusty
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46594
Reviewed By: seemethere
Differential Revision: D24426796
Pulled By: janeyx99
fbshipit-source-id: 428c52893d2d35c1ddd1fd2e65a4b6575f260492
