whisper : remove whisper_load_backends function (#3196)

whisper : remove whisper_load_backends function (#3196)

* whisper : remove whisper_load_backends function

This commit removes the `whisper_load_backends` function, which was used
to load all GGML backends.

The motivation for this change push the responsibility of loading
backends to user applications to give them more control over which
backends to load and when. See the references below for more context.

Resolves: https://github.com/ggml-org/whisper.cpp/issues/3182
Refs: https://github.com/ggml-org/whisper.cpp/pull/3042#issuecomment-2801778733
Refs: https://github.com/ggml-org/whisper.cpp/pull/3042#issuecomment-2801928990

* ruby : add check for rwc is NULL

This commit adds a check to ensure that the `rwc` pointer is not NULL
before attempting to mark its members in the garbage collector.

The motivation for this is an attempt to see if this fixed the CI build
as I'm not able to reproduce the issue locally.

Refs: https://github.com/ggml-org/whisper.cpp/actions/runs/15299612277/job/43036694928?pr=3196