Revert "Allow disabling DroidDoc build."

This reverts commit 875e83165081425e74ce7c999b51c01cac0c876c

Change-Id: I670a1bea555cc7ac8346fba347b9cb40092c3da1
Merge branch 'cm-10.1' of github.com:CyanogenMod/android_build into cm-10.1