comps: Do not install grubby

grubby provides the new-kernel-pkg script, which overrides the behaviour
of the kernel-install script. In addition, grubby causes the following
line

  xen_rm_opts=

to become

  xen_rm_opts

in the GRUB2 configuration file (/boot/grub2/grub.conf) every time
it is invoked, where the trailing equals sign ('=') is removed.
This causes a syntax error to be printed out at every boot.

conf/comps-qubes.xml

@@ -167,7 +167,6 @@
       <packagereq type="mandatory">e2fsprogs</packagereq>
       <packagereq type="mandatory">filesystem</packagereq>
       <packagereq type="mandatory">glibc</packagereq>
-      <packagereq type="mandatory">grubby</packagereq>
       <packagereq type="mandatory">hostname</packagereq>
       <packagereq type="mandatory">initscripts</packagereq>
       <packagereq type="mandatory">iproute</packagereq>