Revert "Sync 'isDefined' in presentation compiler cache."

This reverts commit 3608a8191c91062aa608beb77da930cdc3c8cda7.

We need to revert this because of a possible deadlock at startup between the PC
and the classpath resolution. More information on the related ticket (#1001875).

backport to release/scala-ide-3.0.x

Review by @dragos

Re #1001880
Fixes #1001875