remove code specitic to Go<1.13

The cmdString() function made use of Cmd.String(), which was not available
until Go 1.13.

tfexec/cmd.go

diff --git a/tfexec/cmd.go b/tfexec/cmd.go
index e792dc9..83abd22 100644
--- a/tfexec/cmd.go
+++ b/tfexec/cmd.go
@@ -176,7 +176,7 @@
 	cmd.Env = tf.buildEnv(mergeEnv)
 	cmd.Dir = tf.workingDir
 
-	tf.logger.Printf("[INFO] running Terraform command: %s", cmdString(cmd))
+	tf.logger.Printf("[INFO] running Terraform command: %s", cmd.String())
 
 	return cmd
 }

tfexec/cmdstring.go

diff --git a/tfexec/cmdstring.go b/tfexec/cmdstring.go
deleted file mode 100644
index 72370c4..0000000
--- a/tfexec/cmdstring.go
+++ /dev/null
@@ -1,9 +0,0 @@
-package tfexec
-
-import (
-	"os/exec"
-)
-
-func cmdString(c *exec.Cmd) string {
-	return c.String()
-}