python api_test: Drop use of @LXCPATH@

The python api test script was using @LXCPATH@ for one of its checks.
Now that the lxcpath is exposed by the lxc python module directly, this
can be dropped and api_test.py can now become a simple python file without
needing pre-processing by autoconf.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>