gitmodules: Remove "dirty" from manual-build

Accidental change and we should not have dirty
on the manual-build submodule, but keep it on
images-translated to always check out the latest
images.

.gitmodules

@@ -6,4 +6,3 @@
 [submodule "manual-build"]
 	path = manual-build
 	url = https://git.mahara.org/user-manual/manual-build.git
-	ignore = dirty