serial: mctrl_gpio: Remove unused mctrl_gpio_free

serial: mctrl_gpio: Remove unused mctrl_gpio_free

mctrl_gpio_free() was added in 2014 by
commit 84130aace839 ("tty/serial: Add GPIOLIB helpers for controlling
modem lines")

It does have a comment saying:
  '- * Normally, this function will not be called, as the GPIOs will
   - * be disposed of by the resource management code.'

indeed, it doesn't seem to have been used since it was added.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20250129020048.245529-1-linux@treblig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>