cgraph: remove 'itos'

cgraph: remove 'itos'

This function was unsafe to use in the way described. It relied on semantics
that are not guaranteed under C99. That is, the lifetime extension of a struct
member of an rvalue. This changes under C11 to something that would make this
not problematic. But it is unlikely Graphviz will be able to migrate to C11 in
the foreseeable future as MSVC is lacking C11 support.

Gitlab: closes #2229