rtc: tx4939: Remove driver

CPU support for TX49xx is getting removed, so remove rtc driver for it.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210105140305.141401-9-tsbogend@alpha.franken.de