Revert "Support empty Time inputs"

This reverts commit 617ef956b68b8c3292c44a5782c2365e3513a207.

Conflicts:

  spec/inputs/time_select_input_spec.rb

See https://github.com/justinfrench/formtastic/issues/901 for further information.
Merge pull request #1030 from justinfrench/901-time-select-params

Revert "Support empty Time inputs"