qt, wallet: Remove unused AskPassphraseDialog::Decrypt

Summary:
Co-authored-by: Sehyun Chung <sehyun@berkeley.edu>

This is a backport of core-gui#109 [1/3]
https://github.com/bitcoin-core/gui/pull/109/commits/6e950118a31fd6a85026d934fc6adb6255e47e23

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10737