make: Drop building without driver modules

Driver modules proved to be reliable for a long time. Since support for
not building modules complicates the code and makefiles drop it.

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>

tests/testutils.c

@@ -451,9 +451,7 @@ void virTestCaptureProgramExecChild(const char *const argv[],
     int stdinfd = -1;
     const char *const env[] = {
         "LANG=C",
-# if WITH_DRIVER_MODULES
         "LIBVIRT_DRIVER_DIR=" TEST_DRIVER_DIR,
-# endif
         NULL
     };