Treure submodule backend

Ja no té sentit tenir el submodule del backend, degut al fet que els dos
repositoris són relativament independents.
diff --git a/.gitmodules b/.gitmodules
index 91a3700..e69de29 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +0,0 @@
-[submodule "backend"]
-	path = backend
-	url = https://github.com/delefme/covid-tracability-backend
diff --git a/backend b/backend
deleted file mode 160000
index c16e8d1..0000000
--- a/backend
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit c16e8d15baa438954f63c81372037e5e89af88ea