[TASK] ext:install Improve wording in empty database select

On first installation only existing empty databases can be selected.
The hint for "Existing tables will be removed" is obsolete.

Change-Id: I05c0fb460672b8b4bbffe321945c46d2c7be2b2f
Resolves: #52314
Releases: 6.2
Reviewed-on: https://review.typo3.org/24078
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn