Remove VRBrain board sub-types

[peterbarker]

these were only used for arranging which Notify LED backends to select, so eliminate them in favour of a specific "backend-style" define.

[peterbarker]

I've checked for binary output changes after this PR, and these are the changes:

pbarker@threads:~/rc/ardupilot$ ~/rc/ardupilot/Tools/scripts/filter_size_compare_branches_csv.py /tmp/some.csv  --hide-bin 
------------------------  -----  ----------  ------  ----  -----  -----  ---
Board                     blimp  bootloader  copter  heli  plane  rover  sub
SITL_arm_linux_gnueabihf  0                  0       0     0      0      0
SITL_x86_64_linux_gnu     0                  0       0     0      0      0
VRBrain-v51               -8     *           -16     -8    -8     -16    -8
VRBrain-v52               -16    *           -8      -16   -16    -8     -16
VRBrain-v54               -8     *           -8      0     -8     -8     -8
VRCore-v10                -8     *           -16     -8    -8     -16    -8
VRUBrain-v51              -8     *           -16     -8    -8     -16    -8
------------------------  -----  ----------  ------  ----  -----  -----  ---
pbarker@threads:~/rc/ardupilot$ 

... so the only boards affected are the VRBrain boards.

I've looked with elf-diff, and it's only ROMFS which is changed on those boards (hwdef.dat inclusion)