Simplify our maven-javadoc-plugin configuration, and move both it and maven-source-plugin to the default phase (which is package).

[copybara-service]

Simplify our maven-javadoc-plugin configuration, and move both it and maven-source-plugin to the default phase (which is package).

I doubt that this ends up being a complete no-op in all scenarios (if only because of the phase change), but I didn't set out intending to change any behavior, and my various tests all passed:

• mvn clean javadoc:jar deploy -DskipTests=true -Dmaven.deploy.skip=true (also also a similar test without javadoc:jar, as discussed below)
• mvn clean deploy -Psonatype-oss-release -DskipTests -Drelease -Dmaven.deploy.skip=true (now that it's passing after cl/572327204)
• mvn clean install -DskipTests

As part of this CL, I'm removing javadoc:jar from the command line we use for snapshots. I do this out of fear that it will someday result in "duplicated artifacts" (like cl/571437790), use the wrong configuration, or just run twice—as apparently it does in my test of the main changes from this CL:

[INFO] --- javadoc:3.5.0:jar (default-cli) @ guava ---
[INFO] No previous run data found, generating javadoc.
[INFO] Building jar: /usr/local/google/home/cpovirk/tmp.lS88PuqjZ4/git/guava/target/guava-HEAD-jre-SNAPSHOT-javadoc.jar
...
[INFO] --- javadoc:3.5.0:jar (attach-docs) @ guava ---
[INFO] Configuration changed, re-generating javadoc.
[INFO] Building jar: /usr/local/google/home/cpovirk/tmp.lS88PuqjZ4/git/guava/target/guava-HEAD-jre-SNAPSHOT-javadoc.jar

The new snapshot command appears to run Javadoc exactly once, as desired:

[INFO] --- javadoc:3.5.0:jar (attach-docs) @ guava ---
[INFO] No previous run data found, generating javadoc.

(While in the neighborhood, I considered also investigating a switch from jar to jar-no-fork, similar to what we did for maven-source-plugin, but I didn't bother. FWIW, I didn't notice any duplicate steps, so the switch might not buy us anything for Guava in particular.)

Also, I removed some mentions of org.apache.maven.plugins, which are redundant because that is the default.

RELNOTES=n/a