ofFile::doesFileExist simplified #111

Job	Run time
build-vs2022 (x64)	7m 28s
build-linux64-platforms (linux64, 64gcc6)	4m 28s
build-emscripten (emscripten)	3m 20s
build-vs2022 (ARM64)	7m 12s
build-vs2022 (ARM64EC)	8m 3s
build-msys2 (mingw64)	6m 4s
build-macos (osx, xcode)	1m 25s
rpi-build (armv6l, armhf, arm-linux-gnueabihf, 1)	2m 58s
build-macos (osx, makefiles)	2m 7s
rpi-build (aarch64, arm64, aarch64-linux-gnu, 1)	2m 28s
build-ios-tvos (ios, ios)	3m 35s
	49m 8s

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