[electrum] remove a few unused instance attributes for menus and actions

Summary:
These attributes are unused outside of the scope of the init_menubar method.

Depends on D18311

Test Plan:
start Electrum ABC, check for no error message.

Search for the attribute names in the entire project to confirm they are unused.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18315