cocci: simplify check for trivial format strings

353d84c5 (coccicheck: make transformation for strbuf_addf(sb, "...")
more precise) added a check to avoid transforming calls with format
strings which contain percent signs, as that would change the result.
It uses embedded Python code for that.  Simplify this rule by using the
regular expression matching operator instead.

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

contrib/coccinelle/strbuf.cocci

 @ strbuf_addf_with_format_only @
 expression E;
-constant fmt;
-@@
-  strbuf_addf(E,
-(
-  fmt
-|
-  _(fmt)
-)
-  );
-
-@ script:python @
-fmt << strbuf_addf_with_format_only.fmt;
-@@
-cocci.include_match("%" not in fmt)
-
-@ extends strbuf_addf_with_format_only @
+constant fmt !~ "%";
 @@
 - strbuf_addf
 + strbuf_addstr