[flang] Whether a procedure's interface is explicit or not is not a d… (#82796)

…istinguishing characteristic

We note whether a procedure's interface is explicit or implicit as an
attribute of its characteristics, so that other semantics can be checked
appropriately, but this internal attribute should not be used as a
distinguishing characteristic in itself.

Fixes https://github.com/llvm/llvm-project/issues/81876.