Clean up unused upper_string function
Clean up unused upper_string function

Shows up as a warning at compile time:
```
libairspyhf/src/airspyhf.c:549:13: warning: ‘upper_string’ defined but not used [-Wunused-function]
  549 | static void upper_string(unsigned char *string, size_t len)
      |             ^~~~~~~~~~~~
```

Observed on g++ 9.2.0. After removing the unused function the code compiled without the warning.
airspy/airspyhf