[BUGFIX] Use database name property in admin_get_tables

The admin_get_tables method in the DatabaseConnection
uses the constant instead of the class property.
This delivers wrong results if using multiple
DatabaseConnection objects.

Use the class property instead.

Change-Id: I93e05fdabb53b170782c61ef4ebfa97190cddfb6
Resolves: #49738
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/22021
Reviewed-by: Markus Klein
Tested-by: Markus Klein