Remove "ndk/"-prefixed include path compatibility workarounds

With the way that the native version of `android_ndk_repository` structured / symlinked the NDK into bazel's external repository directory, the way to include things like cpu-features was

```
#include "ndk/sources/android/cpufeatures/cpu-features.h"
```

With the way the Starlark version of `android_ndk_repository` structures the NDK symlinks, the `ndk/` prefix is not needed:
```
#include "sources/android/cpufeatures/cpu-features.h"
```

However this presents a migration issue for projects that use the native version of the rule, so a workaround was introduced to allow including the headers with the `ndk/` prefix (by adding another symlink that results in the previous include path). We should migrate projects and remove this workaround.