Stop trying to fixup 'overloadable' prototypeless functions.

While investigating something else, I discovered that a prototypeless
function with 'overloadable' was having the attribute left on the
declaration, which caused 'ambiguous' call errors later on. This lead to
some confusion.  This patch removes the 'overloadable' attribute from
the declaration and leaves it as prototypeless, instead of trying to
make it variadic.