Remove defunct maybe_restore_shipping_methods

[c-shultz]

Fixes #398

Description

Removes the private maybe_restore_shipping_methods method and all references from the class WC_Subscriptions_Cart. @james-allan has an excellent summary here. The expected nonce field has been renamed since WooCommerce 3.4 in 2018, so this code has been unused for about 7 years. It has no known purpose at this point and contain a call to a long deprecated core function (WC_Customer::calculated_shipping).

Also deprecates the public method maybe_restore_chosen_shipping_method since it is no longer reference internally.

How to test this PR

The code appears to have been related to fixing a corner case when the shipping method is updated on the cart page, but the nonce check has not been passing, so this shouldn't have any impact other than remove a logged PHP warning. I would encourage exploratory testing to double-check that nothing was missed.

At a minimum, I recommend:

• Adding a shortcode cart page
• Adding a subscription product to cart
• Updating the address and shipping method on the cart page

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• Added deprecated functions, hooks or classes to the spreadsheet

[barryhughes]

Minor detail (not essential we change anything, though, as this is a bit of a grey area), but might this be better as:

Suggested change

	wcs_deprecated_function( __METHOD__, 'subscriptions-core 8.1.0', 'The use of this function is no longer recommended and will be removed in a future version.' );
	wcs_deprecated_function( __METHOD__, '7.3.0', 'The use of this function is no longer recommended and will be removed in a future version.' );

Here's my thinking:

• From a user perspective, the product they are using is WooCommerce Subscriptions and I believe we currently expect the next version to be 7.3.0.
• I realize Subscriptions Core can be used elsewhere, not just in the Subscriptions product, but that feels like more of a temporary edge case (in fact, based on some recent discussion, we decided to generally use the version number from the parent plugin for things like @since and @version tags).

[barryhughes]

I left a comment about a possible change, but beyond that this looks great!

• Tests as per the provided instructions.
• I experimented with a few different shipping provider choices, to test for any problems, and couldn't find any.
• Verified this check will not be triggered outside of a shortcode-powered environment.

Approving but not merging so you can review my note re version numbers in deprecation notices, but definitely go ahead with a merge when you are ready.

Thanks!