pthread.h: Remove unused #define, MAX_THREAD_ID
pthread.h: Remove unused #define, MAX_THREAD_ID

BUG=none
TEST=build

Review URL: https://codereview.chromium.org/1724143002

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client