[sphinx] remove source_suffix specification

source_suffix was changed from list to dict in 1.8 but

 source_suffix = {'.rst': 'restructuredtext'}

is default already, so remove it to suppress warnings about that.

Change-Id: I05081784e565286be08d444640bee133a3ff1617

docs/conf.py

@@ -72,10 +72,9 @@
 templates_path = ['_templates']
 
 # The suffix(es) of source filenames.
-# You can specify multiple suffix as a list of string:
+# You can specify multiple suffix as a dict:
 #
-# source_suffix = ['.rst', '.md']
-source_suffix = '.rst'
+# source_suffix = {'.rst': 'restructuredtext'}
 
 # The encoding of source files.
 #