[CMAKE] Refactor warnings for secp256k1
The -Wshadow warning is duplicated from the top level CMakeLists.txt
and can therefore be removed.
The -Wno-nonnull is not useful as it actually doesn't remove any
warning. This also makes it more consistent with autotools build which
doesn't enforce this warning suppression.
The warnings should be the same before and after this patch:
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3798