Remove unreachable code
The change removes the following warning by cython(==0.29.16).
```
[ 1/16] Cythonizing cupy/core/_kernel.pyx
warning: cupy/core/_kernel.pyx:114:4: Unreachable code
```
@@ -111,7 +111,6 @@ cdef str _get_c_type(Arg arg):
return _get_typename(arg.dtype)
else: # indexer
return 'CIndexer<%d>' % arg.ndim
- assert False
cdef str _get_param_c_type(Arg arg, ParameterInfo p):
