Remove DoNotMock annotation in favor of JavaDoc

DoNotMock was removed from error_prone_annotations in 2.1.3, because
there was no enforcement mechanism (which is in google/error-prone#572).
Guava and Trust also depend on error_prone_annotations and are beginning
to use newer versions, so our usage of DoNotMock is causing diamond
dependency problems. This allows us to update to 2.2.0.

The annotations were useful internally; we're solving that in cl/205294089.
diff --git a/build.gradle b/build.gradle
index 46b8c2c..ac175d6 100644
--- a/build.gradle
+++ b/build.gradle
@@ -194,7 +194,7 @@
             epoll_suffix = ":" + osdetector.classifier
         }
         libraries = [
-            errorprone: "com.google.errorprone:error_prone_annotations:2.1.2",
+            errorprone: "com.google.errorprone:error_prone_annotations:2.2.0",
             gson: "com.google.code.gson:gson:2.7",
             guava: "com.google.guava:guava:${guavaVersion}",
             hpack: 'com.twitter:hpack:0.10.1',