Delete variations when product type changes to a non-variable product

[gumgumcode]

Is your feature request related to a problem? Please describe.

The issue isn't exactly with WC Core but fixing it here makes more sense as it simply supports what needs to happen in WC Subscriptions by making a minor change in product_type_changed function without any side-effects in WC Core.

Current function
Proposed changes to the function

The original issue is located in WC Subscriptions: https://github.com/woocommerce/woocommerce-subscriptions/issues/3672 and a potential PR for it is located here: https://github.com/woocommerce/woocommerce-subscriptions/pull/3732.

Describe the solution you'd like

The PR I linked above achieves the following expected behavior:

• Changing product type from variable subscription to variable product (and vice versa) will preserve the variations (will not delete them).
• Changing the product type from variable sub/product to a non-variable product will delete the variations.

If this sounds good, I'll make a new PR in WC Core (moving the code from my PR in WC Subscriptions).

Describe alternatives you've considered

Unhooking the product_type_changed function in WC Core and re-hooking it in WC Subscriptions as explained in the PR linked above but it makes more sense to do it WC Core.