Bug 1041866 - Enable arithmetic optimization in proguard.cfg. r=mfinkle

Arithmetic optimization hasn't been a problem since Donut, so we're well past
the point of supporting it. Optimize away!

mobile/android/config/proguard.cfg

@@ -44,10 +44,6 @@
 # http://proguard.sourceforge.net/manual/examples.html#androidapplication
 #
 
-# Switch off some optimizations that trip older versions of the Dalvik VM.
-
--optimizations !code/simplification/arithmetic
-
 # Keep a fixed source file attribute and all line number tables to get line
 # numbers in the stack traces.
 # You can comment this out if you're not interested in stack traces.