Revert forcing use of single-parameter static_assert()

Some clang versions define static_assert as a macro apparently, so we cannot
redefine it...

This reverts commit 94bfea0ce6a7b4d5641c198d50751748a353df11.
This reverts commit 4e62627831e7457ed60ff87712570065b14b200a.
diff --git a/src/hb-set-digest.hh b/src/hb-set-digest.hh
index 9d38dae..0f9329f 100644
--- a/src/hb-set-digest.hh
+++ b/src/hb-set-digest.hh
@@ -60,8 +60,8 @@
 				     + (mask_bytes >= 16? 1 : 0)
 				     + 0;
 
-  static_assert ((shift < sizeof (hb_codepoint_t) * 8));
-  static_assert ((shift + num_bits <= sizeof (hb_codepoint_t) * 8));
+  static_assert ((shift < sizeof (hb_codepoint_t) * 8), "");
+  static_assert ((shift + num_bits <= sizeof (hb_codepoint_t) * 8), "");
 
   inline void init (void) {
     mask = 0;