Remove linked app from links file when destroying app

tests/hook_pre_delete.bats

@@ -0,0 +1,20 @@
+#!/usr/bin/env bats
+load test_helper
+
+setup() {
+  dokku apps:create my_app >&2
+  dokku "$PLUGIN_COMMAND_PREFIX:create" l >&2
+  dokku "$PLUGIN_COMMAND_PREFIX:link" l my_app >&2
+}
+
+teardown() {
+  dokku "$PLUGIN_COMMAND_PREFIX:unlink" l my_app >&2
+  dokku --force "$PLUGIN_COMMAND_PREFIX:destroy" l >&2
+  rm "$DOKKU_ROOT/my_app" -rf
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:hook:pre-delete) removes app from links file when destroying app" {
+  [[ -n $(< "$PLUGIN_DATA_ROOT/l/LINKS") ]]
+  dokku --force apps:destroy my_app
+  [[ -z $(< "$PLUGIN_DATA_ROOT/l/LINKS") ]]
+}