Remove unused guard

Summary:
PR description:

BITCOIN_QT_TEST is no longer used since switching to autotools build system.

Some historical refs:

https://github.com/bitcoin/bitcoin/pull/807
https://github.com/bitcoin/bitcoin/pull/4241

Backport of Core PR16350

Test Plan:

grep -r BITCOIN_QT_TEST[^_] .
cd build
ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7846