remove unused External from ConvErr enum

Change-Id: I93d653d92466bfaa0bddc8ca5958e1328cf1d6cf
Reviewed-on: https://gerrit.libreoffice.org/34466Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/filter/excel/excform.cxx

@@ -1750,7 +1750,6 @@ void ExcelToSc::SetError( ScFormulaCell &rCell, const ConvErr eErr )
     switch( eErr )
     {
         case ConvErr::Ni:         nInd = FormulaError::UnknownToken; break;
-        case ConvErr::External:   nInd = FormulaError::NoName; break;
         case ConvErr::Count:      nInd = FormulaError::CodeOverflow; break;
         default:                  nInd = FormulaError::NoCode;   // I had no better idea
     }

sc/source/filter/inc/formel.hxx

@@ -45,7 +45,6 @@ enum class ConvErr
 {
     OK = 0,
     Ni,      // unimplemented/unknown opcode occurred
-    External,// excel add-ins are not converted
     Count    // did not get all bytes of formula
 };