build: remove third party submodules

Most of these are extremely common in distributions now, so packagers
don't have much need for our in-tree versions - most don't even use them
as is

With our move to vcpkg for Windows/macOS, we also don't have a need for
them. So time to say goodbye!

Signed-off-by: Seth Flynn <getchoo@tuta.io>

.gitmodules

@@ -1,18 +1,6 @@
-[submodule "libraries/tomlplusplus"]
-	path = libraries/tomlplusplus
-	url = https://github.com/marzer/tomlplusplus.git
 [submodule "libraries/libnbtplusplus"]
 	path = libraries/libnbtplusplus
 	url = https://github.com/PrismLauncher/libnbtplusplus.git
-[submodule "libraries/zlib"]
-	path = libraries/zlib
-	url = https://github.com/madler/zlib.git
-[submodule "libraries/extra-cmake-modules"]
-	path = libraries/extra-cmake-modules
-	url = https://github.com/KDE/extra-cmake-modules
-[submodule "libraries/cmark"]
-	path = libraries/cmark
-	url = https://github.com/commonmark/cmark.git
 [submodule "flatpak/shared-modules"]
 	path = flatpak/shared-modules
 	url = https://github.com/flathub/shared-modules.git