all: Remove `ProviderServerWithResourceIdentity` Checks

[austinvalle]

terraform-plugin-mux version

v0.19.0 (not yet released)

Proposal

After hashicorp/terraform-plugin-go#478, ProviderServerWithResourceIdentity will become part of the main ProviderServer interface. Once that release is available, the temporary interface checks for ProviderServerWithResourceIdentity should be removed from this codebase and just call the downstream ProviderServer directly