ARC: [arcfpga] Get rid of ARC_BOARD_ANGEL4 and ARC_BOARD_ML509

Commit c00bfd97 ("ARC: [arcfpga] Get rid of legacy BVCI latency unit
support") removed the Kconfig symbol ARC_HAS_BVCI_LAT_UNIT. And that
symbol's entry was the only place were the symbols ARC_BOARD_ANGEL4 and
ARC_BOARD_ML509 were used. So ARC_BOARD_ANGEL4 and ARC_BOARD_ML509 can
be removed too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>