[SDL Audio] Refactoring Mix_LoadWAV_RW to use new FS API.

The old API was causing issues with certain file systems, as the `read` property required the execute bit AND the read bit to be set before it would return `true`.

As a side benefit, the new code is simpler and cleaner.
Merge pull request #1894 from jvilk/legacy_fs_remove

[SDL Audio] Refactoring Mix_LoadWAV_RW to use new FS API.