RFR: 8284581: Serial: Remove unused GenCollectedHeap::collect_locked

[Albert Mingkun Yang]

On Fri, 8 Apr 2022 12:58:20 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial change of removing some dead code.
> 
> Test: build

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/8163