dt-bindings: serial: Remove obsolete nxp,lpc1850-uart.txt

nxp,lpc1850-uart.txt binding is already covered by 8250.yaml, so remove
it.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230707221607.1064888-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>