Support `remove_columns`

`def remove_columns(table_name, *column_names)` is defined
in ActiveRecord::ConnectionAdapters::SchemaStatements, which does not
require Oracle enhanced specific implementation.

`def remove_column(table_name, column_name, type = nil, options = {})`
ignores `type` and `options` arguments to avoid ORA-00904 errors

See rails/rails#8267 for Rails change.
This commit should not backport into release14 branch.
Merge pull request #377 from yahonda/support_remove_columns

Support `remove_columns`