[sanitizer] More dead code removal

Summary:
The following functions are only used in tests: `SetEnv`,
`SanitizerSetThreadName`, `SanitizerGetThreadName`. I don't think they are
going to be used in the future, and I propose to get rid of them, and associated
tests and include.

Reviewers: alekseyshl, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: dvyukov, vitalybuka, kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D45838

llvm-svn: 330724