Supplant explicit use of git submodules with repos.conf.

The build system uses 'repos.conf' to drive git submodules implicitly in the background, allowing the package to use submodules
transparently, whether the package is built from a git-controlled source repository or from a distribution archive outside of
git control.

Author: Grant Erickson

.gitmodules

@@ -1,12 +0,0 @@
-[submodule "third_party/nlassert/repo"]
-	path = third_party/nlassert/repo
-	url = https://github.com/nestlabs/nlassert.git
-	branch = master
-[submodule "third_party/nlio/repo"]
-	path = third_party/nlio/repo
-	url = https://github.com/nestlabs/nlio.git
-	branch = master
-[submodule "third_party/nlunit-test/repo"]
-	path = third_party/nlunit-test/repo
-	url = https://github.com/nestlabs/nlunit-test.git
-	branch = master