image_types_phosphor: drop overlay

The overlay image type doesn't have any code behind it and thus
selecting it in IMAGE_FSTYPES would result in unexpected behavior.

An overlay image type still exists but it was renamed to mtd-static
in an earlier refactor, and this cleanup was omitted.

Tested: Built witherspoon and palmetto images
Change-Id: I7eb41afcf25ae2d251bf671dd0e5a18e1064f7c4
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>