Remove attr and libcap dependency from lxc

This is now provided by slapgrid-sr itself. There's no need to compile
it for the software (since it's not possible to compile it as non-root).

component/lxc/buildout.cfg

 [buildout]
 
 extends =
-  ../attr/buildout.cfg
-  ../libcap/buildout.cfg
   ../patch/buildout.cfg
 
 parts = lxc