Don't infer counter_caches from attribute names

This commit fixes issues ##3903 and #8446 in which false positive
detection of counter_cache columns based on column name causes
inconsistent behaviour.

Without this commit, creating an attribute like:

   "#{table_name}_count"

on an ActiveRecord class with a has_many or has_many_through
association to a class mapped to "table_name" was sufficent to
have that column treated as a counter_cache. This was undocumented
behaviour.

After this commit, the only way to create a counter cache is to
specify the `:counter_cache` option on the `belongs_to` side of an
association.