As part of T212461: Update TrustedXFF to use core's StaticArrayWriter implementation (b701bad1f83cc20d51d20885ac276d17d97a11ae, to be specific), @Krinkle removed support for wgTrustedXffFile from TrustedXFF extension. It appears our own config still sets it though (and there's still a copy of the PHP file in MW config).
Per codesearch, nothing else uses the variable, so it can be removed safely.