overlays: Remove hack from uart0 overlay

The uart0 overlay contained a hack to return GPIOs 14 and 15 to inputs
when the UART0 function was moved to alternative pins. This has the
unwanted side effect of claiming GPIOs 14 & 15, preventing them being
used for something else.

See: https://github.com/raspberrypi/linux/issues/2856
     https://www.raspberrypi.org/forums/viewtopic.php?f=98&t=252911



Signed-off-by: Stefan Enge <stefan.enge@escatec.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.org>