statefbk.acker doesn't use _ssmatrix-converted value of inputs

From gh-1099 ; probably needs a similar treatment to `ctrb` there.

pyflakes says:

```
control/statefbk.py:278:5: local variable 'b' is assigned to but never used
```

code with docstring removed:

```
def acker(A, B, poles):
    # Convert the inputs to matrices
    a = _ssmatrix(A)
    b = _ssmatrix(B)

    # Make sure the system is controllable
    ct = ctrb(A, B)
```
