Bug 210280

Our jhbuildrc adds the system paths to PKG_CONFIG_PATH. This is not
necessary, as they are included implicitly by pkg-config, e.g.

$ mkdir /tmp/empty
$ PKG_CONFIG_PATH=/tmp/empty pkg-config --cflags glib-2.0
-I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include

Apart from not being necessary, it is problematic because it makes
impossible to override system libraries in jhbuild-wrapper.

$ PKG_CONFIG_PATH=/home/ntrrgc/Apps/gst-1.10-prefix/lib/pkgconfig/ jhbuild-wrapper --gtk run bash
/webkit/WebKitBuild/DependenciesGTK/Root/lib64/pkgconfig:/webkit/WebKitBuild/DependenciesGTK/Root/lib/pkgconfig:/webkit/WebKitBuild/DependenciesGTK/Root/share/pkgconfig:/usr/share/pkgconfig:/usr/lib64/pkgconfig:/home/ntrrgc/Apps/gst-1.10-prefix/lib/pkgconfig

Note /usr/share/pkgconfig and /usr/lib64/pkgconfig are taking priority
over the user environment.

This patch removes that code, so that the user PKG_CONFIG_PATH takes
priority over the system default paths, while these keep having effect
too as explained before.

$ PKG_CONFIG_PATH=/home/ntrrgc/Apps/gst-1.10-prefix/lib/pkgconfig jhbuild-wrapper --gtk run bash
/webkit/WebKitBuild/DependenciesGTK/Root/lib64/pkgconfig:/webkit/WebKitBuild/DependenciesGTK/Root/lib/pkgconfig:/webkit/WebKitBuild/DependenciesGTK/Root/share/pkgconfig:/home/ntrrgc/Apps/gst-1.10-prefix/lib/pkgconfig