Remove terraform-plugin-mux dependency

[bbasata]

This pull request proposes to remove the terraform-plugin-mux dependency.

This simplifies main.go, reaches parity with the acceptance test provider in TestAccProtoV6ProviderFactories, and means there is one less dependency to keep up with.

(Context: I 💟 DNSimple, I work in Terraform DevEx at HashiCorp, and I am always curious about how people are extending Terraform 😃)

Acceptance test run

This gives no signal on potential regressions, because main() is outside the reach of acceptance tests.

For completeness, here is an acceptance test run. Exactly 1 test fails, unrelated to this change. I imagine it may be something missing in my sandbox environment.

DNSIMPLE_CLEANUP_DOMAINS=true \
DNSIMPLE_ACCOUNT=... \
DNSIMPLE_SANDBOX=true \
DNSIMPLE_TOKEN=... \
DNSIMPLE_CONTACT_ID=... \
DNSIMPLE_REGISTRANT_CHANGE_CONTACT_ID=... \
DNSIMPLE_DOMAIN=... \
DNSIMPLE_REGISTRANT_CHANGE_DOMAIN=... \
  make testacc

=== RUN   TestAccDomainDelegationResource
    domain_delegation_resource_test.go:19: Step 1/4 error: Error running apply: exit status 1

        Error: API returned an error

          with dnsimple_domain_delegation.test,
          on terraform_plugin_test.tf line 12, in resource "dnsimple_domain_delegation" "test":
          12: resource "dnsimple_domain_delegation" "test" {

        Validation failed: missing glue records for ns-998.awsdns-60.net.
--- FAIL: TestAccDomainDelegationResource (1.66s)

Smoke test: build, install, apply

$ make build
==> Checking that code complies with gofmt requirements...
go install

$ cd example
$ terraform apply
╷
│ Warning: Provider development overrides are in effect
│
│ The following provider development overrides are set in the CLI configuration:
│  - dnsimple/dnsimple in /.../.../go/bin
│
│ The behavior may therefore not match any released version of the provider and applying changes may cause the state to become incompatible with published releases.
╵
var.dnsimple_account
  DNSimple Account ID

  Enter a value: 

var.dnsimple_domain
  DNSimple Domain

  Enter a value: gopher.taxi

var.dnsimple_token
  DNSimple API Token

  Enter a value:


Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # dnsimple_zone_record.test-txt will be created
  + resource "dnsimple_zone_record" "test-txt" {
      + id               = (known after apply)
      + name             = "test-tf-txt"
      + name_normalized  = (known after apply)
      + priority         = (known after apply)
      + qualified_name   = (known after apply)
      + ttl              = 3600
      + type             = "TXT"
      + value            = "Hello Terraform!"
      + value_normalized = (known after apply)
      + zone_id          = (known after apply)
      + zone_name        = "gopher.taxi"
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

dnsimple_zone_record.test-txt: Creating...
dnsimple_zone_record.test-txt: Creation complete after 0s [name=test-tf-txt]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

[weppos]

Thanks a lot @bbasata. We'll verify the changes in our upcoming maintenance cycle.