Revert "Fix plugins lookup when not building in the Qt prefix"

This reverts commit 38e06651.

This change is incorrect, as Qt/QCA does not look within $pluginDir/qca/crypto when loading it's plugins.
Instead, it expects to find them within the $pluginDir/crypto/ subdirectory which no longer exists with this change.

While the use of lib/qca/ is non-standard for plugins, existing code within the CI scripts is able to handle this abnormality.
Normal Qt libraries which use plugins also function normally on the CI system.

The change as it stands does not work with Qt 4 and breaks all tests which are reliant upon QCA on the CI system.
As it appears a decision needs to be made on where QCA plugins should live and how they should be loaded i'm reverting this change to allow the tests to pass again.

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