Remove dead code in do_pg_backup_start().

Remove dead code in do_pg_backup_start().

As of commit 39969e2a1, no caller of do_pg_backup_start() passes NULL
for labelfile or tblspcmapfile, nor is it plausible that any would
do so in the future.  Remove the code that coped with that case,
as (a) it's dead and (b) it causes Coverity to bleat about possibly
leaked storage.

While here, do some janitorial work on the function's header comment.