Remove libcurl and libssh2

[pataxis]

These libraries entered the 1.5 release and caused a build error in one of the examples.

Do also clean out some other new libraries that entered this release.

Checklist before requesting a review

• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [ ] I have made corresponding changes to the documentation

[marbali8]

Did you run tests?

[pataxis]

Yes, tests have now run and finished. All examples build and tests pass as expected.