qt5.qtwebkit: Fixes modules src being subtly broken.

b785d4813e introduced breakage in Qt
modules for 5.6 and 5.9, especially visible is Qt Webkit.

This was manifested by having a non-sensical build log where it is using
the top-level `src` attribute as source instead of Qt Webkit's own
source.

Were it not for the `src` top-level attribute (which is a legit
package), the error would have been made obvious by passing `null` to
`src`.

This partily reverts newly introduced way `src` can be passed to a
qtModule, instead relying on extending the `srcs` attrset.

For ZHF #45960

pkgs/development/libraries/qt-5/modules/qtwebkit.nix

@@ -5,8 +5,6 @@
 , bison2, flex, gdb, gperf, perl, pkgconfig, python2, ruby
 , darwin
 , flashplayerFix ? false
-, src ? null
-, version ? null
 }:
 
 let
@@ -35,9 +33,6 @@ qtModule {
 
   cmakeFlags = optionals (lib.versionAtLeast qtbase.version "5.11.0") [ "-DPORT=Qt" ];
 
-  inherit src;
-  inherit version;
-
   __impureHostDeps = optionals (stdenv.isDarwin) [
     "/usr/lib/libicucore.dylib"
   ];