Remove duplicated plugin, set up just above (#230)

Fixes:
[WARNING] Some problems were encountered while building the effective model for jakarta.json:jakarta.json-api:bundle:2.0.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-source-plugin @ line 149, column 21
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.

Signed-off-by: Piotrek Zygielo <piotr@zygielo.pl>

api/pom.xml

diff --git a/api/pom.xml b/api/pom.xml
index c71f578..86fa4bd 100644
--- a/api/pom.xml
+++ b/api/pom.xml
@@ -148,10 +148,6 @@
             </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-source-plugin</artifactId>
-            </plugin>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
                 <configuration>
                     <docfilessubdirs>true</docfilessubdirs>