Fix forms with 24hr datetime inputs being blackholed.

Omit the meridian picker from the secured fields if the form has a 24
hour format hour picker.

Refs #5105
Fix failing test.

meridian is no longer secured by default as the default hour format is
24 hrs.
Merge pull request #5122 from cakephp/meridian-security-component

Fix forms with 24hr datetime inputs being blackholed.