depends: remove FORCE_USE_SYSTEM_CLANG

Summary:

Remove FORCE_USE_SYSTEM_CLANG in favour of always using the system Clang and lld for macOS cross-compilation; rather than downloading precompiled blobs.

Backport of core#30201 and core#30231.

Depends on D17265.

Note the CI update will be done in another diff. It only affects the cross osx build on CI which is currently paused.

Test Plan: Run the guix osx build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17266