[cmake] civetweb: use src tarball and remove dead code

[dpiparo]

in the CMakeLists.txt file.
This is done to reduce the number of clones during the build process.

[github-actions]

Test Results

    11 files      11 suites   1d 13h 40m 59s ⏱️
 3 853 tests  3 853 ✅ 0 💤 0 ❌
37 503 runs  37 503 ✅ 0 💤 0 ❌

Results for commit 1417e61.