Temporarily remove graylog-plugin-pipeline-processor dependency

In order to be able to build the old Graylog project manifest,
this commit temporarily removes the graylog-plugin-pipeline-processor dependency.

Refs Graylog2/graylog2-server#4590
Refs Graylog2/graylog2-server#4592
Refs Graylog2/graylog-project-internal#8

pom.xml

@@ -50,12 +50,6 @@
             <version>${graylog.version}</version>
             <scope>provided</scope>
         </dependency>
-        <dependency>
-            <groupId>org.graylog.plugins</groupId>
-            <artifactId>graylog-plugin-pipeline-processor</artifactId>
-            <version>${graylog.version}</version>
-            <scope>provided</scope>
-        </dependency>
         <dependency>
             <groupId>commons-net</groupId>
             <artifactId>commons-net</artifactId>