Revert "add brcm2708-bcm2710 for raspi 4 support"

This reverts commit 0df9f251. The target
doesn't seem to be supported, the build failed:

  make[1]: Leaving directory '/builds/firmware/site-ffs/gluon/openwrt'
  Please set GLUON_TARGET to a valid target. Gluon supports the following targets:
   * ar71xx-generic
   * ar71xx-tiny
   * ar71xx-nand
   * ath79-generic
   * brcm2708-bcm2708
   * brcm2708-bcm2709
   * ipq40xx-generic
   * ipq806x-generic
   * lantiq-xrx200
   * lantiq-xway
   * mpc85xx-generic
   * mpc85xx-p1020
   * ramips-mt7620
   * ramips-mt7621
   * ramips-mt76x8
   * ramips-rt305x
   * sunxi-cortexa7
   * x86-generic
   * x86-geode
   * x86-legacy
   * x86-64
  Makefile:158: recipe for target 'config' failed