cmake: drop auto-update of submodules

Submodules are auto-updated in two cases:
1. When cmake is called for the first time (and there's no
   src/lib/small/CMakeLists.txt).
2. If git-describe version doesn't match the version stored in the
   VERSION file.

The first case looks okay, but the second one is confusing. It means
that the following sequence of commands does NOT update submodules:

```sh
cmake . # initial configuration, fetches small
cd src/lib/small && git checkout HEAD^ && cd - # checkout small
cmake . # does NOT update small
```

while the following sequence of commands does:

```sh
git commit --allow-empty -m dummy
cmake . # updates small!
```

This looks confusing, inconsistent, and dangerous. Let's drop this code
and let the developers update submodules manually when they need to.

This effectively reverts commit c83eea60
("CMake: automatically update submodules").