Remove python-xlib submodule

- The python-xlib submodule wasn't removed correctly, which caused one of our workflows to fail. We'll reintroduce this submodule later as we now have the umu-sdk which includes build dependenices for it

Author: R1kaB3rN

.gitmodules

@@ -10,6 +10,3 @@
 [submodule "subprojects/unzip"]
 	path = subprojects/unzip
 	url = https://salsa.debian.org/sanvila/unzip.git
-[submodule "subprojects/python-xlib"]
-	path = subprojects/python-xlib
-	url = https://salsa.debian.org/python-team/packages/python-xlib.git