[Cashtab] Remove websocketDisconnectedRefreshInterval

Summary: As per feedback in D15281, splitting out this removal of webdocketDisconnectedRefreshInterval into a separate diff before standardizing the refresh interval in D15281.

Test Plan:

• npm test
• grep -r websocketDisconnectedRefreshInterval src/ with no output
• npm start
• Verify no Suspending wallet update interval while new wallet is activated message upon wallet switch

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15296