remove hgImporterFetchFallback

Summary:
# This diff

Removes a dead config option

# Context

The hg importer is long dead. We can remove this config option as it will never be used anywhere.

Differential Revision: D65491513

fbshipit-source-id: f55c26de87545a680ec76800d810564a2c19ab7e

eden/fs/config/EdenConfig.h

@@ -843,14 +843,6 @@ class EdenConfig : private ConfigSettingManager {
       1024,
       this};
 
-  /**
-   * Whether fetching objects should fall back to hg importer process.
-   */
-  ConfigSetting<bool> hgImporterFetchFallback{
-      "hg:importer-fetch-fallback",
-      true,
-      this};
-
   ConfigSetting<uint32_t> hgActivityBufferSize{
       "hg:activity-buffer-size",
       100,