BF: Removed breaking .nii.gz from remove_ext call

The remove_ext call contained a spurious .nii.gz which was ignored by the old bash-based remove_ext ( e.g. "foo.nii.gz .nii.gz" would return "foo" whereas the new python-based version would return "foo " - that is correctly returning a space-separated list but with the second element being of zero-length. This additional space would break subsequent calls.