trace2: use warning() directly in tr2_dst_malformed_warning()

Let warning() format the message instead of using an intermediate strbuf
for that.  This is shorter, easier to read and avoids an allocation.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2/tr2_dst.c

@@ -166,13 +166,8 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 static void tr2_dst_malformed_warning(struct tr2_dst *dst,
 				      const char *tgt_value)
 {
-	struct strbuf buf = STRBUF_INIT;
-
-	strbuf_addf(&buf, "trace2: unknown value for '%s': '%s'",
-		    tr2_sysenv_display_name(dst->sysenv_var), tgt_value);
-	warning("%s", buf.buf);
-
-	strbuf_release(&buf);
+	warning("trace2: unknown value for '%s': '%s'",
+		tr2_sysenv_display_name(dst->sysenv_var), tgt_value);
 }
 
 int tr2_dst_get_trace_fd(struct tr2_dst *dst)