[eclipse-cdt#327] fix blocking jobs (eclipse-cdt#328)

...by removing synchronized from refreshTreeContentFromLS. The method
will be called from different jobs started by the
DeferredTreeContentManager. The completable future returned by
symbols.thenAcceptAsync cannot join, when the refreshTreeContentFromLS
is synchronized.

fixes eclipse-cdt#327

bundles/org.eclipse.cdt.lsp/src/org/eclipse/cdt/lsp/internal/ui/navigator/SymbolsManager.java

@@ -224,7 +224,7 @@ private Object[] getCompileUnitElements(Object object) {
  return EMPTY;
  }
 
- private synchronized void refreshTreeContentFromLS(CompileUnit compileUnit) {
+ private void refreshTreeContentFromLS(CompileUnit compileUnit) {
  if (compileUnit == null || !compileUnit.isDirty) {
  return;
  }