[ruby/rdoc] lint: Remove unreachable code

(ruby/rdoc#1137)

This is an attempt to utilize RuboCop further.
RuboCop was added in ruby/rdoc@9262fdd43a3a
but only a few rules have been enabled.
I believe we can utilize RuboCop more for better code quality,
especially with Lint cops.
This is the first step to enable other Lint cops.
This commit also exclude some auto generated files.

ruby/rdoc@a53287fce0

lib/rdoc/rdoc.rb

@@ -395,7 +395,6 @@ def parse_file filename
     $stderr.puts e.backtrace.join("\n\t") if $DEBUG_RDOC
 
     raise e
-    nil
   end
 
   ##