Drop unneeded Qt metatype declarations

Inhibition::State, Job::Error, Power::InhibitionType are all registered
with Q_ENUM, thus have automatic coverage.

QList<T> with T being a registered meta type that is also registered
automatically, and int as built-in C++ type is registered.

The explicit declaration macros only add costs to all places using the
header, where the metatype declaration will be only needed by code
using the types in queued connections.