doc: Remove obsolete CHANGES-staging and UPGRADE-staging

doc/CHANGES-staging/README.md

doc/CHANGES-staging/ami_hook_flash.txt

doc/CHANGES-staging/app_mixmonitor_mute_by_id.txt

...ANGES-staging/bridge_builtin_features_beep_on_monitor.txt

doc/CHANGES-staging/cli_channel_display_length_increase.txt

doc/CHANGES-staging/dundi.txt

doc/CHANGES-staging/format_sln_support_for_slin.txt

doc/CHANGES-staging/res_http_media_cache.txt

doc/CHANGES-staging/test.txt

doc/UPGRADE-staging/README.md