x86: remove the rdc subtarget, it was broken for ages and nobody cared

target/linux/x86/Makefile

target/linux/x86/rdc/base-files/etc/config/network

target/linux/x86/rdc/base-files/etc/diag.sh

...linux/x86/rdc/base-files/lib/preinit/05_set_ether_mac_rdc

target/linux/x86/rdc/base-files/lib/upgrade/platform.sh

target/linux/x86/rdc/config-default

target/linux/x86/rdc/profiles/ar525w.mk

target/linux/x86/rdc/profiles/bifferboard.mk

target/linux/x86/rdc/profiles/r8610.mk

target/linux/x86/rdc/profiles/sitecom.mk

target/linux/x86/rdc/target.mk