From docs:
inputMask : QString [QLineEdit]
...
Unset the mask and return to normal QLineEdit operation by passing an empty string ("") or just calling setInputMask() with no arguments.
...

But I get erros using setInputMask "with no arguments" (clang):
lineEdit->setInputMask();

error: too few arguments to function call, expected 1, have 0
lineKeyCode->setInputMask();
~~~~~~~~~~~~~~~~~~~~~~~~~ ^