uci: Remove obsolete and broken plugin

The plugin was apparently broken for years because it uses functions that
don't exist anymore.  It was quite limited anyway, so it was never really
used in OpenWrt to begin with (instead they generate configs in a custom
init script).

scripts/test.sh

@@ -290,7 +290,7 @@ all|alpine|codeql|coverage|sonarcloud|no-dbg|no-testable-ke)
 	CONFIG="--enable-all --disable-android-dns --disable-android-log
 			--disable-kernel-pfroute --disable-keychain
 			--disable-lock-profiler --disable-padlock --disable-fuzzing
-			--disable-osx-attr --disable-tkm --disable-uci
+			--disable-osx-attr --disable-tkm
 			--disable-unwind-backtraces
 			--disable-svc --disable-dbghelp-backtraces --disable-socket-win
 			--disable-kernel-wfp --disable-kernel-iph --disable-winhttp"