remove datafusion testing module

Author: alamb

.gitmodules

@@ -4,7 +4,3 @@
 [submodule "testing"]
 	path = testing
 	url = https://github.com/apache/arrow-testing
-[submodule "datafusion-testing"]
-	path = datafusion-testing
-	url = https://github.com/apache/datafusion-testing.git
-	branch = main