func: don't set func->def in func_lua_new

func->def is supposed to be set by func_new, but func_lua_new sets it
for func_persistent_lua_load to work. Actually, there's no need to do
this, because we can simply pass func_def to func_persistent_lua_load
instead. Let's do this - this is needed to clean up func_def handling
in func_new, which in turn is required to make a copy of a space upgrade
function for read view.

Needed for https://github.com/tarantool/tarantool-ee/issues/163

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring