Don't use CachingAssetAccessor on the web

`CachingAssetAccessor` is used to cache HTTP requests, so we don't waste time requesting the same resource multiple times. When running in a web browser, however, caching is provided automatically by the web browser itself. The extra manual cache is unnecessary, and it's not entirely clear where the request cache SQLite database is even being stored. We should disable it, which should be a simple matter of excluding it from `UnityExternals.cpp` under `__EMSCRIPTEN__`.

This may be the cause of some memory growth seen in [this thread](https://community.cesium.com/t/webgl-memory-accumulation/44906).