linux-initrd: Remove now obsolete #:guile-modules-in-chroot? parameter.

* gnu/build/linux-boot.scm (boot-system): Remove
  #:guile-modules-in-chroot? and related code.
* gnu/system/linux-initrd.scm (base-initrd): Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Remove
  #:guile-modules-in-chroot? argument in 'base-initrd' call.