backend/lightning: don't expose mnemonic words for lightning

[strmci]

This commit ensures the mnemonic words created for lightning wallet are not exposed to frontend.

[strmci]

Plase, also remove the mnemonic from the frontend object :)

Removed.

[strmci]

@Beerosagos @benma I recreated the lightning config without a mnemonic in the handler, PTAL