machine: Remove unnecessary record self-referencing bindings.

'this-machine' and 'this-machine-ssh-configuration' were useless given
that there are no thunked fields.

* gnu/machine.scm (<machine>)[this-machine]: Remove.
* gnu/machine/ssh.scm (<machine-ssh-configuration>)
[this-machine-ssh-configuration]: Remove.

gnu/machine.scm

@@ -33,7 +33,6 @@
 
             machine
             machine?
-            this-machine
 
             machine-operating-system
             machine-environment
@@ -89,10 +88,8 @@
 ;;; Declarations for machines in a deployment.
 ;;;
 
-(define-record-type* <machine> machine
-  make-machine
+(define-record-type* <machine> machine make-machine
   machine?
-  this-machine
   (operating-system %machine-operating-system); <operating-system>
   (environment      machine-environment)      ; symbol
   (configuration    machine-configuration     ; configuration object

gnu/machine/ssh.scm

@@ -74,7 +74,6 @@
 (define-record-type* <machine-ssh-configuration> machine-ssh-configuration
   make-machine-ssh-configuration
   machine-ssh-configuration?
-  this-machine-ssh-configuration
   (host-name      machine-ssh-configuration-host-name)     ; string
   (system         machine-ssh-configuration-system)        ; string
   (build-locally? machine-ssh-configuration-build-locally? ; boolean