Revert "[tsan] Include unistd.h to declare gettid()"

Summary:
This reverts commit de8c4be.

Unfortunately the fix-forward didn't work. Reverting both and will have
Vitaly re-submit.

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Differential Revision: https://phabricator.intern.facebook.com/D60250703

Author: hctim

compiler-rt/test/tsan/debug_alloc_stack.cpp

@@ -9,7 +9,6 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <unistd.h>
 
 #ifndef __APPLE__
 #include <sys/types.h>