Don't set "required" attribute for checkboxes (unless explicitly specified).
Adding it prevents user from submitting form with checkbox unchecked when the "boolean" validation rule is specified for the field.
Closes #3560
remove unused local vars
Merge pull request #1082 from ADmad/bugfix-3560

Don't set "required" attribute for checkboxes (unless explicitly specifi...
Merge pull request #1083 from ceeram/2.3-pmdvars

Remove unused local vars.