Update proguard rules

Author: sunkup

app/proguard-rules.pro

@@ -7,15 +7,8 @@
 
 -dontobfuscate
 
-# ical4j: keep all iCalendar properties/parameters (used via reflection)
--keep class net.fortuna.ical4j.** { *; }
-
-# ical4j needs this
--keep class org.apache.commons.validator.routines.** { *; }
--dontwarn org.apache.commons.validator.routines.**
-
-# okhttp3's internal API - not used by us but apparently still required
--keep class okhttp3.internal.** { *; }
+# okhttp3's internal util is not used by us
+-dontwarn okhttp3.internal.Util
 
 # we are not using those optional features of ical4j
 -dontwarn org.jparsec.** # parser for filter expressions