[C++][R] Remove deprecated checks for old MINGW due to R < 4.2

### Describe the enhancement requested

We have a couple of snippets like:
```
  if(MINGW AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9")
    # This is for RTools 40. We can remove this after we dropped
    # support for R < 4.2. schannel.h in RTools 40 is old.
```

As far as I understand this should not be necessary anymore as we are using R 4.3 minimum.

### Component(s)

C++, R