Remove duplicate test

The feature "serde-std" gets tested along with "std" during the combos
of two loop, no need to run it explicitly.

Author: tcharding

contrib/test.sh

@@ -41,8 +41,6 @@ if [ "$DO_FEATURE_MATRIX" = true ]; then
 		done
     done
 
-    # Other combos
-    cargo test --all --features="serde-std"
 fi
 
 if [ "$DO_SCHEMARS_TESTS" = true ]; then