Removed giflib as webp dependency

[radarhere]

Alternative to #8658

We started mentioning giflib because it is used by build_libwebp - python-pillow/pillow-wheels#98 / https://github.com/multi-build/multibuild/blob/42d761728d141d8462cd9943f4329f12fe62b155/library_builders.sh#L286-L293

However, https://developers.google.com/speed/webp/docs/compiling states

Install the libjpeg, libpng, libtiff and libgif packages, needed to convert between JPEG, PNG, TIFF, GIF and WebP image formats.

We don't ask libwebp to convert between GIF and WebP formats, so the dependency can be removed by replacing build_libwebp with a direct build_simple call.

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cifuzz still appears to be failing because just runs on main - https://github.com/google/oss-fuzz/blob/master/projects/pillow/Dockerfile

If I test one of our old vulnerabilities on oss-fuzz, you can see the convert problem - https://github.com/radarhere/oss-fuzz/actions/runs/12605648048/job/35134631013#step:7:6363

But once I switch to this branch, it goes away - https://github.com/radarhere/oss-fuzz/actions/runs/12605660381