Removed DISCO_F407VG from the official build

It was basically useless, since there's no corresponding target on
mbed.org

Author: Bogdan Marinescu

workspace_tools/build_release.py

@@ -65,7 +65,6 @@
 
     ('LPC11U68',     ('uARM','GCC_ARM','GCC_CR')),
     ('GHI_MBUINO',     ('ARM', 'uARM', 'GCC_ARM')),
-    ('DISCO_F407VG', ('ARM', 'GCC_ARM')),
 )