[cmake] Remove NO_MT flag (#462)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e373733..b374e57 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,7 +38,6 @@
 
 # feel free to discuss these or more with the maintainers
 add_definitions(-DHAVE_OT)
-add_definitions(-DHB_NO_MT)
 add_definitions(-DHB_DISABLE_DEPRECATED)
 
 if (BUILD_SHARED_LIBS)