Purge sfio support, which has been broken for a decade.

The last Perl release that built with -Dusesfio was v5.8.0, and even that
failed many regression tests. Every subsequent release fails to build, and
in the decade that has passed we have had no bug reports about this. So it's
safe to delete all the code. The Configure related code will be purged in a
subsequent commit.

2 references to sfio intentionally remain in fakesdio.h and nostdio.h, as
these appear to be for using its stdio API-compatibility layer.

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