Remove cmake AX_USE_ALSOFT optional for apple platforms

[halx99]

Is your feature request related to a problem? Please describe.

iOS play sounds have issue #2866 with OpenAL.framework which is marked deprecated by Apple few years.

Finally
Since Axmol 2.9.2, on iOS and macOS platforms, the default audio backend has been switched from OpenAL.framework (deprecated by Apple years ago) to openal-soft. The previous CMake option -DAX_USE_ALSOFT has been removed, as it is no longer necessary.

Commit message
Switch default backend on iOS/macOS from OpenAL.framework to openal-soft

• OpenAL.framework has long been deprecated by Apple
• openal-soft is now the enforced default backend for native platforms
• Removed obsolete CMake option -DAX_USE_ALSOFT

Release Notes

• On iOS and macOS, the default audio backend has been changed from OpenAL.framework (deprecated by Apple) to openal-soft.
• The CMake option -DAX_USE_ALSOFT has been removed, as it is no longer required.