Revert "Temporarily #include TObject for genreflex."

This reverts commit e8a9e87a87e226973dcd7ef95f0169e0350cc517.
The TClass is now built after the selection loop.
EmitStreamerInfo() shall not build TClasses to reduce required headers.

Now TClass building happens in CloseStreamerInfoROOTFile() instead, i.e. after
the class selection has happened.
genreflex nonetheless needs a "using namespace std" for ROOT::TMetaUtils::
WriteClassInit / ROOT::TMetaUtils::hasOpaqueTypedef()'s lookup to succeed.
With this only a minimal set of headers is included into genreflex at selection
time; the headers required to do I/O are included afterwards.