Remove faulty fallback for unknown functions

Remove faulty fallback for unknown functions

This code was only used when the required assembly code for unknown function
was missing or intentionally disabled. Because it relied on the implementation
of the compiler to optimize the code in such a way as to not disturb passed in
parameters, the fallback code could not be relied on to work. Removing the
fallback path simplifies the support matrix by making unknown functions
definitely not supported (and will return NULL when queried), whereas with the
fallback the code may work or may crash.