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>

plugins/Makefile.am

@@ -21,10 +21,6 @@ if BUILD_HOTPLUG_PLUGIN
 libplug_la_SOURCES += hotplug.c
 endif
 
-if BUILD_MDEVD_PLUGIN
-libplug_la_SOURCES += mdevd.c
-endif
-
 if BUILD_MODULES_LOAD_PLUGIN
 libplug_la_SOURCES += modules-load.c
 endif
@@ -76,10 +72,6 @@ if BUILD_HOTPLUG_PLUGIN
 pkglib_LTLIBRARIES += hotplug.la
 endif
 
-if BUILD_MDEVD_PLUGIN
-pkglib_LTLIBRARIES += mdevd.la
-endif
-
 if BUILD_MODULES_LOAD_PLUGIN
 pkglib_LTLIBRARIES += modules-load.la
 endif