RFR(T) 8250610: SafepointMechanism::disarm_if_needed() is declared but not used

[Reingruber, Richard]

Hi,

please review this trivial clean-up, which removes the declaration of
SafepointMechanism::disarm_if_needed() (forgotten in JDK-8240918).

Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8250610/webrev.0/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8250610

Testing: Submit repo and nightly regression testing @SAP.

Thanks, Richard.