These submodules were causing failures during recursive git clone
operations. The submodule entries were still registered in the git
tree even though they were removed from .gitmodules, causing:
"fatal: No url found for submodule path 'external/lightning/external/libsodium'"
and similar errors for lowdown.
This removes both submodule references from the git index to resolve
recursive submodule initialization failures (exit code 128).
Changelog-None
c70b70b636