remove code specific to Go<1.14

The TFCache.find() function made use of t.Cleanup(), which was not available
until Go 1.14.

tfexec/internal/testutil/tfcache_find.go

diff --git a/tfexec/internal/testutil/tfcache_find.go b/tfexec/internal/testutil/tfcache_find.go
index 28a249a..eb03bcb 100644
--- a/tfexec/internal/testutil/tfcache_find.go
+++ b/tfexec/internal/testutil/tfcache_find.go
@@ -1,5 +1,3 @@
-// +build go1.14
-
 package testutil
 
 import (

tfexec/internal/testutil/tfcache_find_noop.go

diff --git a/tfexec/internal/testutil/tfcache_find_noop.go b/tfexec/internal/testutil/tfcache_find_noop.go
deleted file mode 100644
index 5b8909d..0000000
--- a/tfexec/internal/testutil/tfcache_find_noop.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// +build !go1.14
-
-package testutil
-
-import (
-	"runtime"
-	"testing"
-
-	"github.com/hashicorp/terraform-exec/tfinstall"
-)
-
-func (tf *TFCache) find(t *testing.T, key string, finder func(dir string) tfinstall.ExecPathFinder) string {
-	panic("not implemented for " + runtime.Version())
-}