Remove '-b multithreaded' from .mvn/maven.config

[sdruzkin]

Description

Remove undocumented maven flag "-b multithreaded" from the .mvn/maven.config
to make the build compatible with Maven 3.9 when not using mvnw.

Motivation and Context

Make the build compatible with Maven 3.9

Impact

No impact

Test Plan

mvn clean install

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== NO RELEASE NOTE ==