Remove unused MJPEG define from libjpeg-turbo GYP file

BUG=chromium:608347
R=msarett@google.com

Review URL: https://codereview.chromium.org/1963293003 .
diff --git a/libjpeg.gyp b/libjpeg.gyp
index 5620479..accd79b 100644
--- a/libjpeg.gyp
+++ b/libjpeg.gyp
@@ -25,7 +25,6 @@
       ],
       'defines': [
         'WITH_SIMD',
-        'MOTION_JPEG_SUPPORTED',
         'NO_GETENV',
       ],
       'sources': [