[unit-test] remove (unnecessary) ENABLE_TEST_MAIN definition (#4367)

tests/unit/test_network_data.cpp

@@ -175,7 +175,6 @@ void TestNetworkDataIterator(void)
 } // namespace NetworkData
 } // namespace ot
 
-#ifdef ENABLE_TEST_MAIN
 int main(void)
 {
     ot::NetworkData::TestNetworkDataIterator();
@@ -183,4 +182,3 @@ int main(void)
     printf("\nAll tests passed\n");
     return 0;
 }
-#endif