ARM: remove u300 platform

The Ericsson U300 platform was one of two ARM929 based SoC platforms for
mobile phones in ST-Ericsson after the merger of Ericsson with ST-NXP
into ST-Ericsson, the other one being the ST Nomadik.

The platform was not widely adopted in Linux based systems and was
replaced with the far superior ST-Ericsson U8500 in 2011, but Linus
Walleij kept maintaining the code for the whole time.

Linus continues to use the Nomadik machine, but decided to drop
u300 from the kernel as part of this year's spring cleaning.
Thanks for having maintained it all these years.

Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/lkml/CACRpkdbJkiHR9FSfJTH_5d_qRU1__dRXHM1TL40iqNRKbGQfrQ@mail.gmail.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>