bankedit.py: remove redundant memory_changed function

[Patronics]

Now that the _cell_changing method handles both cases, memory_changed is redundant.

[kk7ds]

This fails on a radio with an actual index:

ERROR: <function ChirpBankEdit._cell_changing at 0x1223ab370> raised unexpected exception
Traceback (most recent call last):
  File "/Users/dan/Documents/chirp.git/chirp/wxui/common.py", line 642, in run_safe
    return fn(*args, **kwargs)
  File "/Users/dan/Documents/chirp.git/chirp/wxui/bankedit.py", line 225, in _cell_changing
    self._change_memory_index(self.row2mem(row), int(value))
ValueError: invalid literal for int() with base 10: ''

You can use the tests/images/Icom_IC-2820H.img to test. Haven't looked at why, but should be trivial.

[kk7ds]

Slightly more obscure than I was thinking, we get the changing event before the actual value has been edited, so we just need to not try to int() the empty value.