rclone: Rework backend option parsing

This change allows passing no arguments to rclone, using `-o
rclone.args=""`. It is helpful when running rclone remotely via SSH
using a key with a forced command (via `command=` in `authorized_keys`).

internal/backend/rclone/backend.go

@@ -127,8 +127,6 @@ func New(cfg Config, lim limiter.Limiter) (*Backend, error) {
 			return nil, err
 		}
 		args = append(args, a...)
-	} else {
-		args = append(args, "rclone")
 	}
 
 	// then add the arguments
@@ -139,10 +137,6 @@ func New(cfg Config, lim limiter.Limiter) (*Backend, error) {
 		}
 
 		args = append(args, a...)
-	} else {
-		args = append(args,
-			"serve", "restic", "--stdio",
-			"--b2-hard-delete", "--drive-use-trash=false")
 	}
 
 	// finally, add the remote