remove slow tag on e2e test

The test Variable Expansion should fail substituting values in a volume subpath with backticks
takes 12 seconds, it should not be tagged as slow

Author: aojea

test/conformance/testdata/conformance.yaml

@@ -2663,7 +2663,7 @@
   file: test/e2e/common/node/expansion.go
 - testname: VolumeSubpathEnvExpansion, subpath with backticks
   codename: '[sig-node] Variable Expansion should fail substituting values in a volume
-    subpath with backticks [Slow] [Conformance]'
+    subpath with backticks [Conformance]'
   description: Make sure a container's subpath can not be set using an expansion of
     environment variables when backticks are supplied.
   release: v1.19

test/e2e/common/node/expansion.go

@@ -153,7 +153,7 @@ var _ = SIGDescribe("Variable Expansion", func() {
 		Testname: VolumeSubpathEnvExpansion, subpath with backticks
 		Description: Make sure a container's subpath can not be set using an expansion of environment variables when backticks are supplied.
 	*/
-	framework.ConformanceIt("should fail substituting values in a volume subpath with backticks", f.WithSlow(), func(ctx context.Context) {
+	framework.ConformanceIt("should fail substituting values in a volume subpath with backticks", func(ctx context.Context) {
 
 		envVars := []v1.EnvVar{
 			{