Remove implementation of no_constraints in main_adapter

  * Remove skipped tests in `test/ddl_test.rb` that test now unsupported
    (as of PostGIS 2.0) `no_constraints`
  * Remove implementation of `no_constraints` in `main_adapter`
  * Create private `set_dimensions` method in `main_adapter`
  * Refactor `main_adapter#add_column` method to use private
    `add_spatial_column` method