remove problematic 'agmkin', 'agmkout'

remove problematic 'agmkin', 'agmkout'

These functions were introduced in c9b7b0719776e022d760195fae2727b84be82eda to
match `AGMKOUT` and `AGMKIN` (#1212). But there are no exposed prototypes for
these. So the only way to call them is to externally prototype them with a
best-guess type signature and hope Graphviz never changes. This commit removes
them to reduce portability problems.