Remove ‘nss-certs’ package, now part of ‘%base-packages’.

‘nss-certs’ is part of ‘%base-packages’ since Guix commit
fdfd7667c66cf9ce746330f39bcd366e124460e1.

* head-node.scm (packages): Remove ‘nss-certs’.
* build-node.scm (packages): Likewise.

build-node.scm

@@ -63,7 +63,6 @@
   ;; Globally-installed packages.
   (packages (cons* screen
                    openssh                  ;so that 'scp' works
-                   nss-certs
                    %base-packages))
 
   (services

head-node.scm

@@ -399,7 +399,6 @@
   ;; Globally-installed packages.
   (packages (cons* screen
                    openssh                  ;so that 'scp' works
-                   nss-certs                ;for Git checkouts by hpcguix-web
                    %base-packages))
 
   (services