pkg/environment: Remove exec related functions

This commit starts the migration to remove the environment package. It
introduced an abstraction that is not being used, and removing it makes
the code simpler to maintain.

This commit removes all 'exec' related package.

pkg/environment/environment.go

@@ -6,8 +6,6 @@ import (
 	"io/fs"
 	"net"
 	"time"
-
-	"github.com/tkw1536/goprogram/stream"
 )
 
 // Environment represents an environment that a program can run it.
@@ -43,10 +41,6 @@ type Environment interface {
 
 	Listen(network, address string) (net.Listener, error)
 	DialContext(context context.Context, network, address string) (net.Conn, error)
-
-	Executable() (string, error)
-	Exec(ctx context.Context, io stream.IOStream, workdir string, exe string, argv ...string) func() int
-	LookPathAbs(name string) (string, error)
 }
 
 type WritableFile interface {