remove dead code

Summary:
remove dead code

usage of this tunable has been removed but the tunable is still in the list

Reviewed By: clara-9

Differential Revision: D51199020

fbshipit-source-id: 9fc6cb761d3c74db2d54451b93842df04aca7474

eden/mononoke/tunables/src/lib.rs

@@ -324,9 +324,6 @@ pub struct MononokeTunables {
     // Skip backsyncing for empty commits (except mapping changes via extras and merges)
     cross_repo_skip_backsyncing_ordinary_empty_commits: TunableBoolByRepo,
 
-    // Assigning global revs with small gaps
-    global_rev_increment_with_gaps: TunableBool,
-
     // During cross-repo sync, mark a generated changeset as created by lossy conversion if it is
     // See [this post](https://fburl.com/workplace/l5job9po) for context
     // The repo it is tuned by refers to the source repo in the sync