refactor: Remove legacy custombtn and configfile

[rockstorm101]

This PR removes code, which is not standard practice and constitutes a breaking change. Such changes should only be introduced on major version upgrades. However I believe it could be justified in this case and only a minor version upgrade would be needed in the future.

custombtn.txt was deprecated before Printrun 2.x but the code was left. The code didn't even work as expected so this potentially breaking change has a very low chance of affecting any users.
Closes #1236

The configfile function was outdated and only used for handling custombtn.txt so it is safe to remove as well.
Closes #1286

[DivingDuck]

Just check the Windows binary. Is it possible that you delete the function configfile in utils.py by accidentally? I got this error when I try to run Pronterface:

Traceback (most recent call last):
  File "pronterface.py", line 30, in <module>
  File "PyInstaller\loader\pyimod02_importers.py", line 450, in exec_module
  File "printrun\pronterface.py", line 36, in <module>
ImportError: cannot import name 'configfile' from 'printrun.utils' (C:Users\Armin\AppData\Local\Temp\_MEI71082\printrun\utils.pyc)

Edit:
I correct myself. In utils.py line 37 we need to delete in addition configfile from the import list

from .utils import install_locale, setup_logging, dosify, \
    iconfile, configfile, format_time, format_duration, \
    hexcolor_to_float, parse_temperature_report, \
    prepare_command, check_rgb_color, check_rgba_color, compile_file, \
    write_history_to, read_history_from

[rockstorm101]

I correct myself. In utils.py line 37 we need to delete in addition configfile from the import list

Ooops, of course. Thanks for testing. What a silly mistake lol. I'll push a fixed commit now.

[DivingDuck]

Guess, you can merge it. I'll wait for updating the translation in #1464.