remove gltf loader async changes #296

Job	Run time
build (windows-latest)	7m 19s
check-doc	8m 33s
check-cfg	2m 10s
build (ubuntu-latest)	4m 5s
check-bevy-internal-imports	5s
build (macos-latest)	2m 57s
check-missing-examples-in-docs	35s
ci	2m 27s
miri	7m 40s
run-examples-macos-metal	6m 51s
typos	8s
toml	2m 0s
check-missing-features-in-docs	0s
markdownlint	1m 5s
check-compiles	0s
build-wasm-atomics	1m 19s
msrv	1m 44s
build-wasm	56s
	49m 54s

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