Drop trigger based primary key support

[yahonda]

Steps to reproduce

When the nextval branch was planned dropping database link was one of the plans.
The nextval branch plan itself had been canceled, I would like to propose dropping trigger based primary key support from Oracle enhanced adapter in the next major version - 6.0.

Expected behavior

In Rails primary key value is generated by database identity data types such as MySQL auto_increment. Oracle enhanced adapter uses database sequence by default. Even using the sequence explicitly controlled by prefetch_primary_key? is implemented differently from other database adapters.

Actual behavior

Users can generate the primary key value based on the database triggers now.

System configuration

Rails version: The next major version of Rails.

Oracle enhanced adapter version: The next major version of Rails.

Ruby version: Ruby versions supported by the next major version of Rails.

Oracle Database version: 11.2