filter-graph: remove the pipe filter

It's a terrible idea, doesn't work so well (locks up the data-loop when
read is blocked) and a security mightmare. If you really need to pipe
samples through some program, do that somewhere else, like from the
command line with pw-cat and pw-record.

src/modules/module-filter-chain.c

@@ -590,38 +590,6 @@ extern struct spa_handle_factory spa_filter_graph_factory;
  * control from "Control" will be copied to "Notify" and the control value will be
  * dumped into the INFO log.
  *
- * ### Pipe
- *
- * The `pipe` plugin can be used to filter the audio with another application using pipes
- * for sending and receiving the raw audio.
- *
- * The application needs to consume raw float32 samples from stdin and produce filtered
- * float32 samples on stdout.
- *
- * It has an "In" input port and an "Out" output data ports.
- *
- * The node requires a `config` section with extra configuration:
- *
- *\code{.unparsed}
- * filter.graph = {
- *     nodes = [
- *         {
- *             type   = builtin
- *             name   = ...
- *             label  = pipe
- *             config = {
- *                 command = "ffmpeg -f f32le -ac 1 -ar 48000 -blocksize 1024 -fflags nobuffer -i \"pipe:\"  \"-filter:a\" \"loudnorm=I=-18:TP=-3:LRA=4\" -f f32le -ac 1 -ar 48000 \"pipe:\""
- *             }
- *             ...
- *         }
- *     }
- *     ...
- * }
- *\endcode
- *
- * - `command` the command to execute. It should consume samples from stdin and produce
- *             samples on stdout.
- *
  * ### Zeroramp
  *
  * The `zeroramp` plugin can be used to detect unnatural silence parts in the audio