Remove unnecessary declaration of __FlashStringHelper in AceButton.h.
Breaks boards using the ArduinoCore-API, which moved the
__FlashStringHelper to the arduino:: namespace, while polluting
the global namespace anyway with a using arduino::__FlashStringHelper shim in an attempt to provide backwards
compatibility, which doesn't always work.
__FlashStringHelperinAceButton.h.__FlashStringHelperto thearduino::namespace, while polluting the global namespace anyway with ausing arduino::__FlashStringHelpershim in an attempt to provide backwards compatibility, which doesn't always work.