refactor: remove pybind11 as submodule

Author: lterrac

.gitmodules

@@ -2,10 +2,6 @@
 	path = extern/tracr
 	url = https://github.com/Algebraic-Programming/TraCR.git
 	branch = main
-[submodule "extern/pybind11"]
-	path = extern/pybind11
-	url = https://github.com/pybind/pybind11.git
-	branch = stable
 [submodule "extern/HiCR"]
 	path = extern/HiCR
 	url = https://github.com/Algebraic-Programming/HiCR.git