PDFBOX-1990: activate direct encoding of TYPE_BYTE_GRAY lossless images now that test works thanks to fix of PDDeviceGray (PDFBOX-1993 and PDFBOX-1955); removed unused imports in PDDeviceGray.

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1580443 13f79535-47bb-0310-9956-ffa450edef68