Do not register extension points with colcon (#23)

As an independent tool built on colcon's framework, this tool should not
register its extension points as colcon extension points. This should
prevent it from appearing in colcon-devtools output, namely
version-check.

This isn't to say that extension point registration and presence in
colcon-devtools wouldn't provide any benefit, this is an effort to avoid
"kitchen sinking" colcon as a command line tool.

setup.cfg

@@ -51,9 +51,6 @@ markers =
     linter
 
 [options.entry_points]
-colcon_core.extension_point =
-    rosdistro_reviewer.element_analyzer = rosdistro_reviewer.element_analyzer:ElementAnalyzerExtensionPoint
-    rosdistro_reviewer.submitter = rosdistro_reviewer.submitter:ReviewSubmitterExtensionPoint
 console_scripts =
     rosdistro-reviewer = rosdistro_reviewer.command:main
 rosdistro_reviewer.environment_variable =