refactor(typval): change FC_CFUNC abstraction into FC_LUAREF

"cfuncs" was only ever used to wrap luarefs. As vim8script is
finished and will not be developed further, support for "cfuncs"
for other usecases are not planned. This abstraction was immediately
broken anyway in order to get luarefs out of userfuncs again.

Even if a new kind of userfunc needs to be invented in the future,
likely just extending the FC_... flag union directy, instead of
invoking unnecessary heap object and c function pointer indirection,
will be a more straightforward design pattern.

src/nvim/lua/converter.h

@@ -9,14 +9,6 @@
 #include "nvim/eval/typval.h"
 #include "nvim/func_attr.h"
 
-typedef struct {
-  LuaRef func_ref;
-} LuaCallable;
-
-typedef struct {
-  LuaCallable lua_callable;
-} LuaCFunctionState;
-
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "lua/converter.h.generated.h"
 #endif