[electrum] move delete_contacts to ContactList

Summary:
Move another block of code that the MainWindow does not need to be in charge of.

Depends on D14536

Test Plan: In the Contacts tab, add, rename and delete a contact.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14537