core: drop no-longer used C.UTF-8 locale option

Its use has been globbed into the more generic
BR2_NEEDS_HOST_UTF8_LOCALE option now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Julius Kriukas <julius@kriukas.lt>
Cc: Christian Stewart <kidovate@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>