Make fsl_gen_3D work with extension-less images

As I understand, the recommended practice in FSL is to always reference images with out a specific nii, nii.gz or img extension, allowing the code to be agnistic to the particular flavor of NIFTI currently in use.

When useing fsl_deface however, I got an error with fsl_gen_3D

Error: Input image is not readable.

even though the image was present and readable. The problem had been that I had specified the image without an extension, and the check for readability was done directly with the supplied image. I think the solution is simple, just deleting that block, and let any readability issues get caught by the imtest that follows (and that is compatabible with extensionless images filenames).

CC: @falmagro