[avalanche] Don't show the -avasessionkey option in the help

Summary: This option is only intended for testing and should not be used by regular users.

Test Plan:

bitcoind -help

Check it's no longer displayed

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11959