arm: Remove old amlogic support

[manu]

Summary

Remove the port for aml8726.
Kernel config was removed in r346096 and this port was never migrated
to GENERIC.
It is also impossible to obtain such hardware nowadays.

[imp]

I may have one of these boards, but they are difficult to work with. Same comment about documenting.

[manu]

In D27281#609456, @imp wrote:

I may have one of these boards, but they are difficult to work with. Same comment about documenting.

Same comment too about release note being enough :)