Fix accessing metadata with MetaDataHandle when MetadataSvc is present

[m-fila]

BEGINRELEASENOTES

• Fixed missing checks for accessing metadata with MetaDataHandle when used with IOSvc

ENDRELEASENOTES

Fixes for MetaDataHandle interoperability with PodioDataSvc and new MetadataSvc/IOSvc introduced in #215:

• MetaDataHandle::get_optional wasn't checking whether PodioDataSvc or IOSvc is used
• MetaDataHandle::get didn't check validity of optional when used with IOSvc

And some additional cleanup:

• removed constructor with missing definition (unused otherwise that would be an error)
• removed workaround for checking service existence. Gaudi's service by default initializes a service if it's missing, hence the check was always true

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This is PR is orthogonal and doesn't depend on #223