cmake: drop c-ares::c-ares alias

cmake: drop c-ares::c-ares alias

Remove the c-ares::c-ares alias that was causing build failures after
bumping the minimum CMake version:

```
CMake Error at cmake/modules/Findc-ares.cmake:34 (add_library):
  add_library cannot create ALIAS target "c-ares::c-ares" because another
  target with the same name already exists.
Call Stack (most recent call first):
  src/CMakeLists.txt:463 (_find_package)
  src/seastar/cmake/SeastarDependencies.cmake:136 (find_package)
  src/seastar/CMakeLists.txt:395 (seastar_find_dependencies)`
```

The alias was originally added for backward compatibility with Seastar,
but is no longer needed since the updated Seastar submodule no longer
references the c-ares::c-ares target.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>