remove unneeded var and fix disable method

Author: jaydeluca

instrumentation/rxjava/rxjava-2.0/library/src/main/java/io/opentelemetry/instrumentation/rxjava/v2_0/TracingAssembly.java

@@ -93,11 +93,6 @@ public final class TracingAssembly {
   private static Function<? super ParallelFlowable, ? extends ParallelFlowable>
       oldOnParallelAssembly;
 
-  @SuppressWarnings("rawtypes")
-  @GuardedBy("TracingAssembly.class")
-  @Nullable
-  private static Function<? super Observable, ? extends Observable> oldOnObservableAssembly;
-
   @GuardedBy("TracingAssembly.class")
   private static boolean enabled;
 
@@ -298,8 +293,7 @@ private static void disableObservable() {
 
   @GuardedBy("TracingAssembly.class")
   private static void disableObservableAssembly() {
-    RxJavaPlugins.setOnObservableAssembly(oldOnObservableAssembly);
-    oldOnObservableAssembly = null;
+    RxJavaPlugins.setScheduleHandler(null);
   }
 
   @GuardedBy("TracingAssembly.class")

instrumentation/rxjava/rxjava-3.0/library/src/main/java/io/opentelemetry/instrumentation/rxjava/v3_0/TracingAssembly.java

@@ -66,11 +66,6 @@ public final class TracingAssembly {
   private static BiFunction<? super Observable, ? super Observer, ? extends Observer>
       oldOnObservableSubscribe;
 
-  @SuppressWarnings("rawtypes")
-  @GuardedBy("TracingAssembly.class")
-  @Nullable
-  private static Function<? super Observable, ? extends Observable> oldOnObservableAssembly;
-
   @SuppressWarnings("rawtypes")
   @GuardedBy("TracingAssembly.class")
   @Nullable
@@ -300,8 +295,7 @@ private static void disableObservable() {
 
   @GuardedBy("TracingAssembly.class")
   private static void disableObservableAssembly() {
-    RxJavaPlugins.setOnObservableAssembly(oldOnObservableAssembly);
-    oldOnObservableAssembly = null;
+    RxJavaPlugins.setScheduleHandler(null);
   }
 
   @GuardedBy("TracingAssembly.class")

instrumentation/rxjava/rxjava-3.1.1/library/src/main/java/io/opentelemetry/instrumentation/rxjava/v3_1_1/TracingAssembly.java

@@ -66,11 +66,6 @@ public final class TracingAssembly {
   private static BiFunction<? super Observable, ? super Observer, ? extends Observer>
       oldOnObservableSubscribe;
 
-  @SuppressWarnings("rawtypes")
-  @GuardedBy("TracingAssembly.class")
-  @Nullable
-  private static Function<? super Observable, ? extends Observable> oldOnObservableAssembly;
-
   @SuppressWarnings("rawtypes")
   @GuardedBy("TracingAssembly.class")
   @Nullable
@@ -300,8 +295,7 @@ private static void disableObservable() {
 
   @GuardedBy("TracingAssembly.class")
   private static void disableObservableAssembly() {
-    RxJavaPlugins.setOnObservableAssembly(oldOnObservableAssembly);
-    oldOnObservableAssembly = null;
+    RxJavaPlugins.setScheduleHandler(null);
   }
 
   @GuardedBy("TracingAssembly.class")