Drop references to NIX_MANFIESTS_DIR and NIX_BUILD_HOOK

Neither of these have been supported in ~years.

t/Makefile.am

@@ -8,10 +8,8 @@ TESTS_ENVIRONMENT = \
   NIX_REMOTE_SYSTEMS=					\
   NIX_CONF_DIR="$(abs_builddir)/nix/etc/nix"		\
   NIX_STATE_DIR="$(abs_builddir)/nix/var/nix"		\
-  NIX_MANIFESTS_DIR="$(abs_builddir)/nix/var/nix/manifests"	\
   NIX_STORE_DIR="$(abs_builddir)/nix/store"			\
   NIX_LOG_DIR="$(abs_builddir)/nix/var/log/nix"		\
-  NIX_BUILD_HOOK=					\
   PGHOST=/tmp	\
   PERL5LIB="$(srcdir):$(abs_top_srcdir)/src/lib:$$PERL5LIB"	\
   PYTHONPATH= \