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