Delete redundant pdom options for Dialog's CloseButton.

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For phetsims/models-of-the-hydrogen-atom#67 ...

In Dialog.ts:

    const closeButton = new CloseButton( {
...
      // pdom
      tagName: 'button',
      innerContent: SunStrings.a11y.closeStringProperty,

The pdom options are redundant and confusing. tagName is set in superclass ButtonNode. innerContent is set as accessibleName in CloseButton.

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@jessegreenberg and I discussed this in Slack, so assigning to him to review. Close if OK.

[jessegreenberg]

Great, thanks!

My understanding is we can delete this a11y string from sun now too, so I removed it in 340c658. If @jbphet says otherwise I will revert that.