gnu: webkitgtk-2.4: Update to 2.8.9, build with gtk3.

* gnu/packages/webkit.scm (webkitgtk-2.4): Update to 2.8.9, build with gtk3.
  Move flex and which to 'native-inputs'.  Remove #:configure-flags.
  (webkitgtk/gtk+-2): New variable.
* gnu/packages/gnucash.scm (gnucash): Use webkitgtk/gtk+-2.
* gnu/packages/patches/webkitgtk-2.4.8-gmutexlocker.patch: Remove file.
* gnu-system.am (dist_patch_DATA): Remove it.

Co-authored-by: Feng Shu <tumashu@163.com>