remove the itix theme submodule

The theme will be reintegrated in this repository with its history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YNHsWYTXPThURz6TbVS1j3

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "themes/itix"]
-	path = themes/itix
-	url = https://github.com/nmasse-itix/hugo-theme-itix.git

themes/itix

@@ -1 +0,0 @@
-Subproject commit 7d4207e94daec3d26b1667d113394fb058c4ad8d