Remove unused GenTree flags (dotnet/coreclr#19840)

Remove unused GenTree flags (dotnet/coreclr#19840)

GTF_IND_ARR_LEN was set by the importer in minopts/debug mode and used only by value numbering, which does not run in minopts/debug mode.

GTF_FLD_NULLCHECK was also set by the importer and not used anywhere. fgMorphField has its own opinion about when an explicit null check is needed.

Commit migrated from https://github.com/dotnet/coreclr/commit/1d885d32db50e8ee931d0d9fbd1081bfbb7ce271