CLEANUP: h3: suppress by default stdout traces

H3_DEBUG definition is removed from h3.c similarly to the commit
  d96361b2703a6364c1116af76016f09807b4c65b
  CLEANUP: qpack: suppress by default stdout traces

Also, a plain fprintf in h3_snd_buf has been replaced to be conditional
to the H3_DEBUG definition.

These changes reduces the default output on stdout with QUIC traffic.
diff --git a/src/h3.c b/src/h3.c
index 7fbd483..711ba23 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -33,8 +33,6 @@
 #include <haproxy/tools.h>
 #include <haproxy/xprt_quic.h>
 
-#define DEBUG_H3
-
 #if defined(DEBUG_H3)
 #define h3_debug_printf fprintf
 #define h3_debug_hexdump debug_hexdump
@@ -651,7 +649,7 @@
 	int32_t idx;
 	int ret;
 
-	fprintf(stderr, "%s\n", __func__);
+	h3_debug_printf(stderr, "%s\n", __func__);
 
 	htx = htx_from_buf(buf);