ArmPlatformPkg/ArmJunoPkg: only have 1 PCD for the FDT

Juno doesn't have lots of DTB files in NOR flash, it only has 1 file,
called "board.dtb" and the motherboard configuration makes the right
choice about which DTB file gets written as board.dtb in NOR.

The code attempts to select which DTB it should use based on the board
variant or configuration.  And this doesn't work because those DTB files
aren't present in NOR flash.

So remove the DTB variants and only load board.dtb.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>