remove some unecessary crates from downgrade_dependencies

nix:0.4.2 is no longer a dependency
synstructure:0.12.0 is no longer required to build

maint/downgrade_dependencies

@@ -9,6 +9,4 @@ cargo +nightly update -Z minimal-versions
 cargo update \
       -p crc32fast \
       -p zeroize_derive:1.1.1 \
-      -p env_logger:0.5.0 \
-      -p synstructure:0.12.0 \
-      -p nix:0.4.2
+      -p env_logger:0.5.0