Drop mdevd plugin, not needed since notify:s6 support was added

For an example how to set it up, see the finit-skel repository:

https://github.com/troglobit/finit-skel/blob/main/skel/etc/finit.d/available/mdevd.conf

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config.md

@@ -551,7 +551,7 @@ option:
     (must be >3) on then command line, Finit provides the following
     syntax (`%n` is replaced by Finit with then descriptor number):
 
-        service notify:s6 mdevd -C -O 4 -D %n
+        service [S12345789] notify:s6 mdevd -O 4 -D %n
 
 [sd_notify()]: https://www.freedesktop.org/software/systemd/man/sd_notify.html
 [s6 expect]:   https://skarnet.org/software/s6/notifywhenup.html
@@ -563,6 +563,16 @@ then service's ready condition which other services can depend on:
     $ initctl -v cond get service/mdevd/ready
     on
 
+This can be used to synchronize the start of another run/task/service:
+
+    task [S] <service/mdevd/ready> @root:root mdevd-coldplug
+
+Finit waits for `mdevd` to notify it, before starting `mdevd-coldplug`.
+Notice how both start in runlevel S, and the coldplug task only runs in
+S.  When the system moves to runlevel 2 (the default), coldplug is no
+longer part of the running configuration (`initctl show`), this is to
+ensure that coldplug is not called more than once.
+
 >  For a detailed description of conditions, and how to debug them,
 >  see the [Finit Conditions](conditions.md) document.
 

doc/plugins.md

@@ -52,9 +52,9 @@ For your convenience a set of *optional* plugins are available:
 
 * *hotplug.so*: Setup and start either udev or mdev hotplug daemon, if
   available.  Enabled by default.
-
-* *mdevd.so*: Set up and start mdevd (mdev replacement).  Enabling this
-  also changes how `hotplug.so` plugin operates.  _Optional plugin._
+  
+  > See the [Services](config.md#services) section in the configuration
+  > guide for an example how to run `mdevd`, alternative to plain mdev.
 
 * *rtc.so*: Restore and save system clock from/to RTC on boot/halt.
   Enabled by default.