runtime-linux: Drop file-descriptor docs (covered by the command-line API)

It's unclear how these apply to runtimes APIs that are not based on
the command line / execve, and the functionality is covered by the
more tightly scoped LISTEN_FDS wording in the command-line docs.

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

runtime-linux.md

@@ -1,11 +1,5 @@
 # Linux Runtime
 
-## File descriptors
-
-By default, only the `stdin`, `stdout` and `stderr` file descriptors are kept open for the application by the runtime.
-The runtime MAY pass additional file descriptors to the application to support features such as [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html).
-Some of the file descriptors MAY be redirected to `/dev/null` even though they are open.
-
 ## Dev symbolic links
 
 After the container has `/proc` mounted, the following standard symlinks MUST be setup within `/dev/` for the io.