[CMake][tsan] Remove --sysroot=.
rL254966 added `--sysroot=.` to prevent accidental including system headers. It caused hassle to FreeBSD (D17383)/NetBSD. The next problem is that we want to include `features.h` (usually `/usr/include/features.h`) to detect `__GLIBC__`.
At this point it seems that `--sysroot=.` adds lots of inconvenience so we disable it for now. If there is a better way preventing accidental system header inclusion we can consider it again.