doc: Remove bits about having to create /gnu/store manually.

* doc/guix.texi (Build Environment Setup): Remove bit about creating
  /gnu/store.  This is done automatically since 15ddeff5.

doc/guix.texi

@@ -383,15 +383,6 @@ Bash syntax and the @code{shadow} commands):
   done
 @end example
 
-The @file{/gnu/store} directory (or whichever was specified with the
-@code{--with-store-dir} option) must have ownership and permissions as
-follows:
-
-@example
-# chgrp guix-builder /gnu/store
-# chmod 1775 /gnu/store
-@end example
-
 @noindent
 The @code{guix-daemon} program may then be run as @code{root} with: