Drop unneeded Qt metatype declarations

KAboutData, KAboutLicense, KAboutPerson, KAboutComponent, KPluginMetaData,
all have a Q_GADGET macro and thus have automatic coverage.

KJob::Unit, QLocale::FormatType are both registered with Q_ENUM, thus
also have automatic coverage.

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.