Distribution: Remove unneeded setup_requires

With the new project layout, `pyyaml` is not needed anymore for parsing
setup.py.

setup.py

@@ -45,8 +45,6 @@ setup(
     scripts=['bin/yamllint'],
     package_data={'yamllint': ['conf/*.yml']},
     install_requires=['pyyaml'],
-    setup_requires=['pyyaml'],  # importing `yamllint` (for APP_NAME etc.)
-                                # requires importing `yaml`
     tests_require=['nose'],
     test_suite='nose.collector',
 )