[WebRTC] Fix broken target for avif_decode_fuzzer

https://bugs.webkit.org/show_bug.cgi?id=270936
<rdar://124556987>

Reviewed by Said Abou-Hallawa.

* Source/WebCore/PAL/ThirdParty/libavif/libavif.xcodeproj/project.pbxproj:
- Clean up the project by removing many duplicate folders.
- Remove the android_jni folder since it was deleted in 275928@main.
(avif_decode_fuzzer target):
- Fix avif_decode_fuzzer by removing repro_fuzz.cc from the Sources
  list.  This fixes the issue described by the bug.
* Source/WebCore/PAL/ThirdParty/libavif/tests/oss-fuzz/avif_decode_fuzzer.cc:
(LLVMFuzzerTestOneInput):
- Add back declaration that was originally added in 272156@main, but
  removed in 275291@main.  This fixes a -Wmissing-prototypes compiler
  warning.

Canonical link: https://commits.webkit.org/276113@main