remove workspace env var and not set it to an empty string. (#388)

Setting the environment variable to an empty string confuses the
terraform binary as seen below:

```
TF_WORKSPACE= terraform init -no-color -input=false -get=false -upgrade=false -backend=false

Error: Terraform Cloud returned an unexpected error

invalid value for workspace
```

With this patch the `TF_WORKSPACE` will not exist and avoids the above
problem. 

tfexec/cmd.go

diff --git a/tfexec/cmd.go b/tfexec/cmd.go
index 083dc4d..5e16032 100644
--- a/tfexec/cmd.go
+++ b/tfexec/cmd.go
@@ -169,7 +169,7 @@
 	env[automationEnvVar] = "1"
 
 	// force usage of workspace methods for switching
-	env[workspaceEnvVar] = ""
+	delete(env, workspaceEnvVar)
 
 	if tf.disablePluginTLS {
 		env[disablePluginTLSEnvVar] = "1"

tfexec/cmd_test.go

diff --git a/tfexec/cmd_test.go b/tfexec/cmd_test.go
index 5aa6fae..c86dae3 100644
--- a/tfexec/cmd_test.go
+++ b/tfexec/cmd_test.go
@@ -46,7 +46,6 @@
 		"TF_LOG_CORE=",
 		"TF_LOG_PATH=",
 		"TF_LOG_PROVIDER=",
-		"TF_WORKSPACE=",
 	}
 }