Cleanup IN_FBCODE logic

[NicolasHug]

#8647 introduced a new way of determining whether the code is running internally. We should remove the existing

vision/test/common_utils.py

Line 29 in 7eb09df

IN_FBCODE = os.environ.get("IN_FBCODE_TORCHVISION") == "1"

to avoid any confusion. (That one is only used in tests).