ARM: rmobile: Zap RCAR_GEN3_EXTRAM_BOOT

This Kconfig option is not used on any board, so drop it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>