Remove accidentally committed makefile change.

asio/src/Makefile.msc

diff --git a/asio/src/Makefile.msc b/asio/src/Makefile.msc
index 1192e59..8f66356 100644
--- a/asio/src/Makefile.msc
+++ b/asio/src/Makefile.msc
@@ -5,7 +5,7 @@
 COMMON_CXXFLAGS = -nologo -EHac -GR -I. -I../include
 
 !ifdef WARNINGS
-WARNINGS_CXXFLAGS = -W4 -wd4512 -wd4447 -DASIO_ENABLE_HANDLER_TRACKING
+WARNINGS_CXXFLAGS = -W4 -wd4512 -wd4447
 !endif
 
 !ifdef STATICRTL