MAINT: io: Remove a few unreachable return statements.

This PR fixes the following warnings that occur when building scipy:

```
Processing scipy/io/matlab/mio5_utils.pyx
warning: mio5_utils.pyx:296:16: Unreachable code
warning: mio5_utils.pyx:475:12: Unreachable code
Processing scipy/io/matlab/streams.pyx
warning: streams.pyx:85:12: Unreachable code
warning: streams.pyx:304:12: Unreachable code
warning: streams.pyx:319:12: Unreachable code
```