[CMAKE] Remove useless dependency for the check-rpcauth target

Summary:
The DEPENDS property of a custom target instructs cmake to bring the
files up to date prior to running the command.
The share/rpcauth.py is not generated but is a source file, then
having it in the DEPENDS makes no sense.

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja check-rpcauth

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4113