remove sfio versioning mechanism

remove sfio versioning mechanism

The sfio library versioned its format structure, _sffmt_s. This allowed multiple
format structures of differing versions to exist in memory at once,¹ with sfio
able to distinguish between a structure created by itself vs a structure created
by a different version of sfio. In theory, this allows multiple versions of the
sfio library to be loaded at once.

In practice, Graphviz does not take advantage of any of this. In fact, it is not
possible to do this within Graphviz as SFIO_VERSION has not been bumped when
lib/sfio is changed. It still claims to be compatible with a version developed
in 2001.

This commit removes the versioning mechanism entirely, in order to both reduce
maintenance overhead as well as remove the runtime cost associated with managing
this data. The latter is trivial in terms of runtime, but it is also work that
has no purpose.

¹ This kind of design often also allows you to serialize a structure to disk or
  network to later be read back in by a potentially differing version of the
  library. In the case of sfio however, this use case was not possible because
  the structure contains function pointers.