Fix does not work dropdown menu some browser

[tenkoma]

The version selection menu does not work well with some browsers.
Previously, I sent a pull request to fix similar defects.

• Mobile Safari(iPad) - Tap does not expand the list.
• Chrome 57, Firefox 52(Surface Pro 3) - Mouseover does not expand the list.

Dropdown reimplemented in CSS (import my commit from cakephp/docs#4430, cakephp/docs#4442, cakephp/docs#4449), and Unnecessary code deleted.

• Focus support for versions menu ... I implemented the behavior in CSS so delete it.
• Bootstrap Hover Dropdown ... I confirmed that it is used only in the version selection menu.
• data-hover="dropdown" ... This attribute was not used by both JavaScript and CSS, so delete it.

Tested in the following environment.

• macOS X Sierra
  • Safari 10, Firefox 52, Chrome 57
• Windows 10 (Surfece Pro 3)
  • IE11, Edge14, Firefox 52, Chrome 57
• iPad
  • Mobile Safari (iOS 10)
• iPhone 6S Plus
  • Mobile Safari (iOS 10) ... Menu are not hidden when the width is small.