CLN: fix cython build warning
```
warning: pandas/_libs/tslib.pyx:724:49: Buffer unpacking not optimized away.
warning: pandas/_libs/tslib.pyx:724:49: Buffer unpacking not optimized away.
```

the error is due to a cdef inline function which uses ndarray[object]. The function has one caller, and it isn't in any perf-critical inner-loop, so the inline isn't important. Remove it and avoid warning during cythonization.
