Remove recursive submodules

Author: alexey-milovidov

.gitmodules

@@ -1,6 +0,0 @@
-[submodule "cpp/submodules/parquet-testing"]
-	path = cpp/submodules/parquet-testing
-	url = https://github.com/apache/parquet-testing.git
-[submodule "testing"]
-	path = testing
-	url = https://github.com/apache/arrow-testing