Revert "Avoid formatting debug message uselessly"

This reverts commit 925a72628b226203e94400da8d43889003804eae.

Reason: introduced an ABI change in the maintenance branch.
diff --git a/include/polarssl/debug.h b/include/polarssl/debug.h
index 406cfa8..2dd89a2 100644
--- a/include/polarssl/debug.h
+++ b/include/polarssl/debug.h
@@ -57,10 +57,7 @@
 
 
 #define SSL_DEBUG_MSG( level, args )                    \
-    do {                                                \
-        if( level <= debug_threshold )                  \
-            debug_print_msg_free( ssl, level, __FILE__, __LINE__, debug_fmt args ); \
-    } while( 0 )
+    debug_print_msg_free( ssl, level, __FILE__, __LINE__, debug_fmt args );
 
 #define SSL_DEBUG_RET( level, text, ret )                \
     debug_print_ret( ssl, level, __FILE__, __LINE__, text, ret );
@@ -150,8 +147,6 @@
                       const char *text, const x509_crt *crt );
 #endif
 
-extern int debug_threshold;
-
 #ifdef __cplusplus
 }
 #endif