Remove the `IndifferentHash` initializer about sinatra HOT 3 OPEN

Now, having re-read the description and example a few times, I also feel it's probably a mistake, a bug, introduced by confusing this API with something like https://api.rubyonrails.org/classes/ActiveModel/AttributeAssignment.html.

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So, I support removing.

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Wow, what a mess. Sorry y'all. Looks like I was aiming to make it work like ActiveSupport::HashWithIndifferentAccess (i.e. initialize an indifferent hash with normalized entries) but maybe erroneously thought that Hash#initialize expects the same kind of argument (when it actually expects a default value object)? So as discussed in #1948 calling super is the wrong thing to do here; I should have either implemented initializing logic a la ActiveSupport::HashWithIndifferentAccess#initialize or simply omitted the initializer entirely. Since we already removed the misleading documentation for the former behavior I support removing the initalizer as well.

Bad design, bad execution, bad documentation... again, my apologies.

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