tsan: remove the stats subsystem

I don't think the stat subsystem was ever used since tsan
development in 2012. But it adds lots of code and this
effectively dead code needs to be updated if the runtime
code changes, which adds maintanance cost for no benefit.
Normal profiler usually gives enough info and that info
is more trustworthy.
Remove the stats subsystem.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D106276