remove git submodules

.gitmodules

-[submodule "linux"]
-	path = linux
-	url = git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
-[submodule "u-boot"]
-	path = u-boot
-	url = https://github.com/mntmn/u-boot
-[submodule "reform"]
-	path = reform
-	url = https://source.mntmn.com/MNT/reform
-[submodule "reform-userland"]
-	path = reform-userland
-	url = https://source.mntmn.com/MNT/reform-userland.git