This is a tab box. Try the following calls sequence:
CommandManager::GetCommandTab (by name) -> CommandTab::CommandTabBoxes -> CommandTab::RemoveCommandTabBox.
I actually just ran into this and resolved it outside of Solidworks.
Build this batch or manually do this and Solidworks will rebuild your tab list when you launch it next.
Dim reg_del As RegistryKey
reg_del = Registry.CurrentUser.OpenSubKey("Software\Solidworks\SOLIDWORKS 2015\User Interface\", True)
Catch ex As Exception