Fix Currency.unregister before the currency table is loaded

[Shaisolaris]

Money::Currency.unregister reads @table directly, so it raises when it is the first call that touches the currency table, for example in an initializer right after require "money":

require "money"
Money::Currency.unregister(:usd)
# NoMethodError: undefined method `delete' for nil

register and the lookup methods all go through table, which loads the currencies on first use. This change makes unregister use table as well. When a currency is actually removed, it now also drops the cached instance and clears the iso_numeric index, as register does, so later lookups do not see stale entries.

Tests:

• New .unregister specs in spec/money/currency_spec.rb. The two that start with an unloaded table fail on main with NoMethodError and pass with the change. The iso_numeric one passes on both and is there as a regression guard.
• rspec: 505 examples, 0 failures locally on Ruby 3.3.
• I could not run RuboCop locally with the project's plugins.

A CHANGELOG entry is added under Unreleased.

[sunny]

Thank you, looks good! (Apart from a lint error.)