MDL-76054 environment: Remove the php-xmlrpc extension check

MDL-76054 environment: Remove the php-xmlrpc extension check

For Moodle 4.1 and up, the php-xmlrpc is not needed anymore:
- All the MNet stuff has been moved to use php library (MDL-76055).
- The webservice/xmlrpc has been moved from core to contrib (MDL-76052).

So we just remove the check here. Starting with 4.1, it's not
needed for any core functionality.

Note that the string has been removed, and not deprecated, because it's
a non-generic string, not belonging to core/moodle main lang file, and
hardly reused ever. That fits with the allowed deletions, not requiring
any deprecation.

Of course, the lang removal only has been applied to master (4.1dev).
Older branches still keep it and will be used when checking < 4.1
upgradability.