This removes the "FSops" naming scheme, which was meant to mirror the
"RWops" naming scheme, which was also recently removed from SDL3.
The build system defines (`SDL_FSOPS_POSIX`, etc) and the source code
filenames retain this, because there's already things using the word
"filesystem" that might overlap (for example, lots of things have a
unique "SDL_sysfilesystem.c", to query base dirs, etc, but almost
everything uses the POSIX "SDL_sysfsops.c" source code.
Fixes#9288.