Don't output usage.txt to the source tree.

Change-Id: I5bcb7282f8104f092a7f0aafdbf8fe3572a2100f
diff --git a/proguard.flags b/proguard.flags
index 19f746a..39784b1 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -18,4 +18,3 @@
 }
 
 -verbose
--printusage usage.txt