sfio sfvscanf, sfprint: remove support for '%n' format specifier

Matthew Fernandezrequested to merge
smattr/graphviz:smattr/96730dab-2ae9-4eec-919d-a09aea41a43c into main

These internal implementations are only used by GVPR. In the GVPR programming language, this format specifier is rejected. There is also no way to construct a pointer even if there were a way to use "%n". This change squashes numerous compiler warnings.

Gitlab: #1998 (closed)

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