Remove DBG GcInfo encoder/decoder (dotnet/coreclr#6374)

Remove DBG GcInfo encoder/decoder (dotnet/coreclr#6374)

This change removes the redundant set of GcInfo encoder/decoder
in the CoreCLR tree called Debug encoder/decoders.

These components are expected to be are well-tested versions
for verification, but are not actively used.
They cause additional overhead wrt maintaining two versions of
GcInfo encoder/decoder as GcInfo format changes.

Commit migrated from https://github.com/dotnet/coreclr/commit/d1697c6f8b6e6d1f06b4d734fabd3b328e1dc724