Delete `fgArgInfo::ArgsComplete():hasStackArgs` local var. (dotnet/coreclr#27350)

Delete `fgArgInfo::ArgsComplete():hasStackArgs` local var. (dotnet/coreclr#27350)

`fgArgInfo` has a field with the same name and the same function. The duplication happened because the local var was added in 2011 and the struct field was added in 2014.

PVS warning:  The 'hasStackArgs' local variable possesses the same name as one of the class members, which can result in a confusion. morph.cpp 1247

Commit migrated from https://github.com/dotnet/coreclr/commit/c9b92bcf062c1b57d836fdcf2e0d45c275423c13