add methods to remove pinned version;

Author: azeng-netflix

hollow-diff-ui/src/main/java/com/netflix/hollow/history/ui/HollowHistoryUI.java

@@ -299,6 +299,10 @@ public String getHeaderDisplayPinnedVersion() {
         return commonHeadersDisplayMap.get(HEADER_DISPLAY_PINNED_VERSION);
     }
 
+    public void removeHeaderDisplayPinnedVersion() {
+        this.commonHeadersDisplayMap.remove(HEADER_DISPLAY_PINNED_VERSION);
+    }
+
     public void setHeaderDisplayPinnedVersion(long version) {
         this.commonHeadersDisplayMap.put(HEADER_DISPLAY_PINNED_VERSION, Long.toString(version));
     }

hollow-explorer-ui/src/main/java/com/netflix/hollow/explorer/ui/HollowExplorerUI.java

@@ -142,10 +142,14 @@ public String getHeaderDisplayPinnedVersion() {
         return headerDisplayMap.get(HEADER_DISPLAY_PINNED_VERSION);
     }
 
-    public void setHeaderDisplayPinnedVersion(long version) {
+    public void setHeaderDisplayPinnedVersion(Long version) {
         this.headerDisplayMap.put(HEADER_DISPLAY_PINNED_VERSION, Long.toString(version));
     }
 
+    public void removeHeaderDisplayPinnedVersion() {
+        this.headerDisplayMap.remove(HEADER_DISPLAY_PINNED_VERSION);
+    }
+
     public void addToHeaderDisplayMap(String key, String value) {
         headerDisplayMap.put(key, value);
     }