Remove forwarding methods. See long commit message for details.

There are two resons for removing it:

First, OFPlugin does not need forwarding anymore. Second is that
forwarding is broken in both, the GNU and the Apple runtime.

In GNU libobjc, objc_msg_sendv is implemented using __builtin_apply,
which is broken on many platforms, including x86_64. If forwarding is
used, the application will just crash. To work around that, I'd need to
parse the type encoding and use libffi to call the method instead of
using objc_msg_sendv.

Now the Apple runtime has a similar problem: There is no objc_msgSendv
for PPC64 and x86_64 as it's only in ObjC1 and on ARM (iPhone), it's
broken (most likely because the iPhone uses only ObjC2 - I was confused
that objc_msgSendv was even in the libobjc there). So I'd need to write
an ASM implementation for these 3.

Writing those 3 ASM implementations (or 5, so we don't depend on ObjC1
stuff on PPC32 and x86 as well) wouldn't be a problem, but there is a
problem the GNU libobjc and the Apple runtime got in common, which
originates from the early ObjC implementations:

forward:: and performv:: were only designed to return scalar types. But
today, it's possible to return floats, structs and unions as well. What
Apple and GNU use here is a very hacky workaround and it's just luck
that it works. forward:: and performv:: both return an id (Apple) or
void* (GNU). forward:: is called by the runtime if you called a method
that is not implemented. The compiler does not know at compile time
that it is not implemented, therefore expects a float as a return. On
x86, floats are returned in sp0. The runtime now notices that the
called method is not implemented and calls forward::. Forward then
calls performv:: to call the right method. The method returns a float
and stores it in sp0. Remember that both, forward:: and performv::
return an id / void*. performv:: returns now and after that, forward::
returns. The return of those was always put into eax, as that's how
scalar values are returned on x86. The original caller of the method
does not expect any return value in eax, but in sp0. This works, as
no code touched sp0. However, you can not rely on sp0 not being
touched. It's just luck that the compiler generates code that does not
touch sp0.

While this works for forwarding due to the ABI on x86 (and the ABIs on
many other platforms allow this hack as well), this fails if you call
performv:: directly on a method returning a float. In this case, the
compiler does not expect a return value in sp0, but in eax, as
performv:: is expected to return id / void*. Therefore the bogus value
in eax will be casted to float and the result will be useless.

This is why I decided to remove forwarding and performv:: from libobjfw
for now. If I encounter a situation where I need forwarding, I'm going
to implement it in a sane way and NOT the objc way. The forwarding
methods this commit removes did it the objc way, which is IMO just
wrong. (That way was ok back then when you only had scalar return
types, but today you're not limited to scalar return types anymore.)