beta.6 updates mediasoup 3.9.0 which has a new Meson build system
That new build system does a pretty good job cleaning compiler leftovers on its
own, so theoretically our cleanup code isn't needed anymore.
Amend history:
build/packages-template/bbb-webrtc-sfu/build.sh -> v2.6.0-beta.6 to beta.7
... drop build artifacts from compiling mediasoup. This reduces package
size from ~ 54 MB to ~ 12 MB.
Additionally this drops the `npm rebuild` command from the after-install
script. It should not be necessary to recompile stuff during
installation. To ensure a clean `node_modules` directory, it will be
cleaned in the before-install script.
