refactor: remove unused Blockly.onContextMenu_

[maribethb]

The basics

• I branched from goog_module
• My pull request is against goog_module
• My code follows the style guide
• My code is presented in the form suggested in the module
  conversion guide
• I have run npm test.

The details

Resolves

Part of #5026 and #5208

Proposed Changes

Remove Blockly.onContextMenu_ from blockly.js

This function appears to be totally unused. It is private, so external developers should not be calling it. The functionality is already separately implemented in inject.js. Therefore, this function can just be removed and not moved or aliased.

Additional Information

Tested manually that right-clicking while focused on an html input still does not suppress the native context menu.

Also, this removes the dependency on Blockly.utils as the only other code depending on it is in Blockly.onKeyDown which is moved in #5476 . Therefore, that PR should be submitted first to keep the dependency graph correct.