ARM: da850_am18xxevm: Remove config and references

ARM: da850_am18xxevm: Remove config and references

This board has been abandonded for a while.  I do not have this hardware
and nobody has appeared to notice and/or care that it's orphaned.
Since much of this code is inserted into the da850evm code base, removing
it from support will make da850evm support easier going forward.
This patch removes the da850_am18xxevm_defconfig, README references to this
board and the reference to this board from the MAINTAINERS list.

Signed-off-by: Adam Ford <aford173@gmail.com>