libmodsecurity: remove dependency on valgrind

valgrind is only supported on certain platforms (it notably lacks support for
armv6l), so needs to be excluded as a dependency on unsupported platforms. In
this case, it turns out that valgrind is not currently used at all, so I
removed it entirely. libmodsecurity can optionally use valgrind to run
tests, but we don't run the tests at all.

pkgs/tools/security/libmodsecurity/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, fetchFromGitHub
 , autoreconfHook, bison, flex, pkg-config
 , curl, geoip, libmaxminddb, libxml2, lmdb, lua, pcre
-, ssdeep, valgrind, yajl
+, ssdeep, yajl
 , nixosTests
 }:
 
@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [ autoreconfHook bison flex pkg-config ];
-  buildInputs = [ curl geoip libmaxminddb libxml2 lmdb lua pcre ssdeep valgrind yajl ];
+  buildInputs = [ curl geoip libmaxminddb libxml2 lmdb lua pcre ssdeep yajl ];
 
   outputs = [ "out" "dev" ];