fix(pg-template): omit PkColumn(pk int64) method in *SelectSQL

[mackee]

This pull request includes changes to the SQL query generation for PostgreSQL and MySQL within the _example directory. The main focus is on removing unnecessary methods and refining template conditions.

Refinement of SQL query generation:

• The PkColumn method was removed because PostgreSQL's implementation does not use LastInsertId to retrieve the inserted row unlike MySQL. Since LastInsertId returns an int64, the PkColumn method was needed for MySQL; however, PostgreSQL uses RETURNING and therefore does not require the PkColumn method.
• _example/postgresql/accounts.gen.go: Removed the PkColumn method from the accountSelectSQL struct.
• _example/postgresql/groups.gen.go: Removed the PkColumn method from the groupSelectSQL struct.
• _example/postgresql/identities.gen.go: Removed the PkColumn method from the identitySelectSQL struct.

Template condition improvements:

• template/select_column.tmpl: Updated the condition to generate the PkColumn method only for MySQL by adding a check for the dialect.