Remove arrow as a gitsubmodule since it is provided by conan.

c2b20158 · Remy Prechelt · 343f711b · c2b20158 · d613aa68 · d613aa68
Commit c2b20158 authored 4 years ago by Remy Prechelt
--- a/.gitmodules
+++ b/.gitmodules
@@ -12,8 +12,4 @@
 	path = modules/conex
 	url = ../../AirShowerPhysics/cxroot.git
 	branch = master
-    shallow = true
-[submodule "externals/arrow"]
-	path = externals/arrow
-	url = https://github.com/apache/arrow
 	shallow = true
--- a/arrow @ d613aa68
+++ b/arrow @ d613aa68
-Subproject commit d613aa68789288d3503dfbd8376a41f2d28b6c9d