Revert "checking in the abstractions for valid type checking:"

This reverts commit c321b309a9a90bbfa0912832c11b3fef52e71840.

Conflicts:
	activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
	activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

Reason: failing test

  1) Error:
test_valid_column(ActiveRecord::ConnectionAdapters::SQLite3AdapterTest):
NoMethodError: undefined method `column' for
    test/cases/adapters/sqlite3/sqlite3_adapter_test.rb:29:in
`test_valid_column'
Revert "Make valid_type? public"

This reverts commit 5d528f835e871f5f9d4b68e8a81cfbe900b7e718.

Relates to f8c8ad5 which is also getting reverted due to failing test.