gnu: texinfo: Remove unneeded dependency on xz.

The impetus for this is that XZ 5.2.2 requires pthreads.  However, when
using the bootstrap GCC, using '-pthread' doesn't work because it tries
to refer to /nix/store/eeeeee-glibc-2.18.  XZ eventually fails to build
with undefined references to pthread symbols.

* gnu/packages/texinfo.scm (texinfo)[inputs]: Remove XZ, which was
unneeded.

gnu/packages/texinfo.scm

@@ -43,7 +43,6 @@ (define-public texinfo
     (build-system gnu-build-system)
     (native-inputs `(("procps" ,procps)))  ;one of the tests needs pgrep
     (inputs `(("ncurses" ,ncurses)
-              ("xz" ,xz)
               ("perl" ,perl)))
 
     (native-search-paths