Disable class diagrams.
Disable class diagrams.

Turning on this Doxygen setting (CLASS_DIAGRAMS) requires the
user to install graphviz, and since there is currently only
one inheritance diagram generated by Doxygen, it should be turned
off.

Tested:
Works on Mac.

Change-Id: I58555fc87b94f4d29e8f27f9896b574b9518c73c

google/scene_lab,google/scene_lab