[NEXUS-6027] Copy fileSizesInDirectory from FileUtils to NEXUS5612DefaultFSLocalRepositoryStorageTest (the only place was used)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-6027] Copy methods from FileUtils to DefaultFSLocalRepositoryStorage (the only place where they were used)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-6027] Remove deprecated FileUtils

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>