windows ci : removed fullbench-dll from appveyor

the test is broken
previous version was too Windows-specific.
It needs to be either refactored to be cross-platform,
or be removed if it doesn't bring any value
(which is indeed unclear).
diff --git a/appveyor.yml b/appveyor.yml
index 0f9142b..7d6ca99 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -9,7 +9,7 @@
     - COMPILER: "gcc"
       HOST:     "mingw"
       PLATFORM: "x64"
-      SCRIPT:   "make allzstd MOREFLAGS=-static && make -C tests test-symbols fullbench-dll fullbench-lib"
+      SCRIPT:   "make allzstd MOREFLAGS=-static && make -C tests test-symbols fullbench-lib"
       ARTIFACT: "true"
       BUILD:    "true"
     - COMPILER: "gcc"