Revert WebP support due to broken builds

This reverts commits 2010fbb0e6, a361ce47a1, and d004bb275a.

platform/default/webp_reader.cpp

@@ -1,27 +0,0 @@
-#include <mbgl/util/image.hpp>
-#include <mbgl/util/premultiply.hpp>
-#include <mbgl/platform/log.hpp>
-
-extern "C"
-{
-#include <webp/decode.h>
-}
-
-namespace mbgl {
-
-PremultipliedImage decodeWebP(const uint8_t* data, size_t size) {
-    int width = 0, height = 0;
-    if (WebPGetInfo(data, size, &width, &height) == 0) {
-        throw std::runtime_error("failed to retrieve WebP basic header information");
-    }
-
-    std::unique_ptr<uint8_t[]> webp(WebPDecodeRGBA(data, size, &width, &height));
-    if (!webp) {
-        throw std::runtime_error("failed to decode WebP data");
-    }
-
-    UnassociatedImage image { size_t(width), size_t(height), std::move(webp) };
-    return util::premultiply(std::move(image));
-}
-
-} // namespace mbgl