Update TODO for Rails v5 [ci skip]

Author: metaskills

RAILS5-TODO.md

@@ -1,13 +1,4 @@
 
-## Rails v5.0
-
-* Does the schema cache serialize properly since we conform to that now?
-* Can we use `OPTIMIZE FOR UNKNOWN`
-  - http://sqlblog.com/blogs/aaron_bertrand/archive/2011/09/17/bad-habits-to-kick-using-exec-instead-of-sp-executesql.aspx
-  - http://stackoverflow.com/questions/24016199/sql-server-stored-procedure-become-very-slow-raw-sql-query-is-still-very-fast
-  - https://blogs.msdn.microsoft.com/sqlprogrammability/2008/11/26/optimize-for-unknown-a-little-known-sql-server-2008-feature/
-
-
 ## Rails v5.1
 
 * BIGINT PK support. https://github.com/rails/rails/pull/26266