Remove unnecessary 'add_dependencies' commands

If 'target_link_libraries' used, no need to add dependency explicitly.

From documentation:

  If a library name matches that of another target in the project a dependency
  will automatically be added in the build system to make sure the library
  being linked is up-to-date before the target links.
Merge pull request #370 from ruslo/remove.dependencies

Remove unnecessary 'add_dependencies' commands