Stop mocking up backports.lzma on readthedocs

For some reason mocking up this module is no longer necessary,
in fact it currently breaks documentation builds when mocked up.

Signed-off-by: Nikola Forró <nforro@redhat.com>

docs/conf.py

@@ -282,4 +282,4 @@
 # This value contains a list of modules to be mocked up. This is useful
 # when some external dependencies are not met at build time and break
 # the building process.
-autodoc_mock_imports = ['backports.lzma', 'rpm', 'koji']
+autodoc_mock_imports = ['rpm', 'koji']