RFR: 8249528: Remove obsolete comment in G1RootProcessor::process_java_roots

[Sangheon Kim]

On Tue, 6 Apr 2021 07:55:04 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Updating the obsolete comment and some trivial change of removing dead code in related files.

> ### Webrevs
> 
>     * 00: [Full](https://webrevs.openjdk.java.net/?repo=jdk&pr=3352&range=00) ([2c7a0894](https://git.openjdk.java.net/jdk/pull/3352/files/2c7a0894a575b158c67f8879cae1ac0c6cb2c299))

Looks good.

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PR: https://git.openjdk.java.net/jdk/pull/3352