vte3-gi test: remove needless code

The code is needless because of the following two reasons:

  1. "case when" usage to dispatch by class is wrong.
  2. respond_to? accepts both String and Symbol.

For 1.:

    case "".class
    when String
      p 1
    else
      p 2
    end
    # => 2

    String === ""       # => true
    String === "".class # => false

For 2.:

    "".respond_to?(:empty?)  # => true
    "".respond_to?("empty?") # => true

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