QTgaFile: support TGA 1.0 format images without footer signature

Remove the mandatory TRUEVISION-XFILE footer signature check in the
QTgaFile constructor. The footer is a TGA 2.0 extension and is not
needed for image decoding — readImage() only uses the header and pixel
data. This fixes TGA 1.0 (original TGA) files being incorrectly rejected
by canRead(), which also prevented QImageReader from falling back to
alternative TGA plugins such as KDE's kimg_tga.so.

Change-Id: Ibd3ad5db22fb13ae098c387bf903897946430164
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>