Remove redundant pwallet nullptr check
Summary: This is the last part of Core PR10286 : https://github.com/bitcoin/bitcoin/pull/10286/commits/89f03120a02690cff8399d77c979169355bf9cae
Test Plan:
make check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D3177