[Documentation] Removing "Doctrine Mapping Types" ...

... in favor of https://www.doctrine-project.org/projects/doctrine-dbal/en/3.8/reference/types.html#reference

Page: https://www.doctrine-project.org/projects/doctrine-orm/en/2.19/reference/basic-mapping.html#doctrine-mapping-types

As announced in doctrine/dbal#6336 (comment) , the goal is to remove this duplicated type information from ORM and replace it with a link to DBAL.

In doctrine/dbal#6341 , I'm adding any detail which I'm deleting here to the DBAL.

docs/en/reference/basic-mapping.rst

@@ -311,7 +311,6 @@ mapping from a PHP string to a SQL VARCHAR (or VARCHAR2 etc.
 depending on the RDBMS brand). Here is a quick overview of the
 built-in mapping types:
 
--  ``string``: Type that maps a SQL VARCHAR to a PHP string.
 -  ``integer``: Type that maps a SQL INT to a PHP integer.
 -  ``smallint``: Type that maps a database SMALLINT to a PHP
    integer.