test: update TimeTest::testToDatabase()

[kenjis]

Description
See #6461

There was 1 failure:

1) CodeIgniter\I18n\TimeTest::testToDatabase
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'۲۰۱۷-۰۱-۱۲ ۰۰:۰۰:۰۰'
+'2017-01-12 00:00:00'

/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/I18n/TimeTest.php:1169
phpvfscomposer:///home/runner/work/CodeIgniter4/CodeIgniter4/vendor/phpunit/phpunit/phpunit:97

https://github.com/codeigniter4/CodeIgniter4/runs/8238507830?check_suite_focus=true

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[kenjis]

I would like to remove Time::toDatabase(), because it is exactly the same as __toString() now.

[datamweb]

Removing Time::toDatabase() seems logical. MGatner was always in his favor, you should see if he agrees with us or not.