[macos] Remove support of duplicated formats from macOS

Currently GSTPlatform and OSXPlatform supports duplicated formats: H.264, MP3, AAC and AIFF. H.264 support on GSTPlatform on macOS is limited to only one stream. There is no need to support multiple formats in both platforms. AIFF should be only supported in GSTPlatform and MP3, AAC and H.264 only supported in OSXPlatform.

Also, support for custom URI scheme needs to be added to OSXPlatform, so we can support playback from JARs.