Avoid calling libjxr's JxrDecApp as we currently don't package it.
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -119,8 +119,6 @@ def image_from_data(data):
     i = QImage()
     if not i.loadFromData(data):
         q = what(None, data)
-        if q == 'jxr':
-            return load_jxr_data(data)
         raise NotImage('Not a valid image (detected type: {})'.format(q))
     return i
 
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -644,11 +644,6 @@ def test():  # {{{
     despeckle_image(img)
     remove_borders_from_image(img)
     image_to_data(img, fmt='GIF')
-    raw = subprocess.Popen([get_exe_path('JxrDecApp'), '-h'],
-                           creationflags=subprocess.DETACHED_PROCESS if iswindows else 0,
-                           stdout=subprocess.PIPE).stdout.read()
-    if b'JPEG XR Decoder Utility' not in raw:
-        raise SystemExit('Failed to run JxrDecApp')
 # }}}