Remove unused method ConfigurationMixin.get_file_contents()

Unused since its introduction in
f999060.

Author: jdufresne

news/50f14abd-68a3-476f-9523-b6c5c86e3c68.trivial.rst

@@ -46,8 +46,3 @@ def tmpfile(self, contents):
         yield path
 
         os.remove(path)
-
-    @staticmethod
-    def get_file_contents(path):
-        with open(path) as f:
-            return f.read()