xfce4-session: Remove unneeded pkg_postinst

The pkg_postinst inherited from previous versions of this recipe no
longer seems necessary; no error message seen at runtime in a quick
test.  As it made the generated packages unnecessarily
machine-specific, remove this post-install step. Based on the ensuring
discussion, this supersedes the original patch [1].

[1] http://lists.openembedded.org/pipermail/openembedded-devel/2015-February/100454.html



Signed-off-by: Ash Charles <ashcharles@gmail.com>
Tested-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>