remove plugin-specific Jenkinsfile

Jenkinsfile

@@ -1,17 +0,0 @@
-pipeline {
-    agent any
-
-    stages {
-        stage('phpstan') {
-            steps {
-                sh """
-                docker run --pull=always \
-						-v ${env.WORKSPACE}:/src/tt-rss/plugins/plugin \
-						--workdir /src/tt-rss \
-						--rm cthulhoo/ttrss-fpm-pgsql-static:latest \
-						php81 ./vendor/bin/phpstan analyse plugins/plugin
-                """
-            }
-        }
-    }
-}