Skip inferring variable type from default values

[dbanck]

This PR removes the concept of inferring types from default values via AttributeValue.

See Radek's comment below for more information.

UX

Before

After

---

• Related to Variable with empty list causes "No declaration found" on submodules vscode-terraform#1592

[radeksimko]

Given the conversation we had with TF Core team on this topic in Slack, I think we should consider entirely binning the concept of inferring type based on value - i.e. remove AsTypeOf entirely, because the behaviour of inferring the type based on default is not something present in Terraform.

We may revisit the treatment of default and generally evaluation of expressions as part of #132 or hashicorp/terraform-ls#495 but for now I think we should bin it.

We currently only use it for variable defaults anyway, so if we don't end up using it at all then we should treat it as dead code worth removing.

It does make our completion less helpful but it may as well just push module authors towards being more explicit and we may create further pressure as part of hashicorp/terraform-ls#1476