[nullsafe] Restrict usage of StrictNullsafe annotations to currently supported modes

Summary:
We support only class level annotations for now. We will add more when
we support more.

Reviewed By: artempyanykh

Differential Revision: D18036213

fbshipit-source-id: 44791318e

infer/annotations/src/main/java/com/facebook/infer/annotation/NullsafeStrict.java

@@ -20,5 +20,5 @@ import java.lang.annotation.Target;
  * practice).
  */
 @Retention(RetentionPolicy.CLASS)
-@Target({ElementType.TYPE, ElementType.FIELD, ElementType.CONSTRUCTOR, ElementType.METHOD})
+@Target({ElementType.TYPE})
 public @interface NullsafeStrict {}