roadmap: Fix roadmap item released in 3.4

Support for race-free repacking was already in 3.4 [1].

[1] https://www.gerritcodereview.com/3.4.html#jgit-changes

Change-Id: I7cb3527b5df8b584706f8b4fe28df1d65eeee964
diff --git a/pages/site/roadmap.md b/pages/site/roadmap.md
index 84600d6..485b8af 100644
--- a/pages/site/roadmap.md
+++ b/pages/site/roadmap.md
@@ -33,7 +33,6 @@
 * UX
   * Composable SubmitRules
 * Switch language level to Java 11
-* Support for race-free zero-downtime pruning of git packed objects
 
 ## Upcoming plugin improvements related to Gerrit 3.5
 * New plugin: Allow group deletion