RFR: 8351821: VMManagementImpl.c avoid switching off warnings

[Kevin Walls]

On Fri, 14 Mar 2025 10:06:32 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

> We switched off unused-variable warnings for the file VMManagementImpl.c, this should be avoided.
> This came up in [JDK-8351542](https://bugs.openjdk.org/browse/JDK-8351542) ,

We know GetOptionalSupport can't realistically fail, unless we call it with a bad pointer (we don't).
This location is called once only on startup.
We have not checked the return value since introduced in jdk5...  So can't really object to not saving that value in a temp that we ignore, and simplifying the Makefile a little!

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Marked as reviewed by kevinw (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24052#pullrequestreview-2686805647