ARCH: ARM: shmobile: Remove ag5evm board support

Remove support for the sh73a0 based ag5evm board.

The sh73a0 SoC is continued to be supported using
the kzm9g board.

Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>