cmFindPackageCommand: Drop dead code

The `cmFindPackageFileList` instances always constructed w/ the only
parameter. The boolean flag is always `true` (default value).

Also, `cmFindPackageCommand::InitialPass` adds an empty string
to the `cmFindPackageCommand::SearchPathSuffixes` vector. Meaning
that `cmFindPackageCommand::CheckDirectory()` gonna be called for
the suffix-less path, so `cmFindPackageFileList::Visit` may call
only `cmFindPackageCommand::SearchDirectory` and get the same result.