[lldb] Return StringRef from PluginInterface::GetPluginName

There is no reason why this function should be returning a ConstString.

While modifying these files, I also fixed several instances where
GetPluginName and GetPluginNameStatic were returning different strings.

I am not changing the return type of GetPluginNameStatic in this patch, as that
would necessitate additional changes, and this patch is big enough as it is.

Differential Revision: https://reviews.llvm.org/D111877

lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

@@ -633,11 +633,6 @@ size_t ObjectFileELF::GetModuleSpecifications(
   return specs.GetSize() - initial_count;
 }
 
-// PluginInterface protocol
-lldb_private::ConstString ObjectFileELF::GetPluginName() {
-  return GetPluginNameStatic();
-}
-
 // ObjectFile protocol
 
 ObjectFileELF::ObjectFileELF(const lldb::ModuleSP &module_sp,