cleanup(spanner): tidy up spanner backup/restore samples (#5874)
cleanup(spanner): tidy up spanner backup/restore samples (#5874)

This is mostly to ease the upcoming merge of backup/restore
samples for PITR and CMEK, which include some general fixes
like eliminating unused using declarations, adding missing
newlines, and correcting tag names.  Make those changes here
first so that the future merges are all about the features.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp