Remove useless pg_strdup() operations.

split_to_stringlist() doesn't modify its first argument nor expect it
to remain valid after exit, so there's no need to duplicate the optarg
string at the call sites.  Per Coverity.  (This has been wrong all along,
but commit 052cc223 changed the useless calls from "strdup" to
"pg_strdup", which apparently made Coverity think it's a new bug.
It's not, but it's also not worth back-patching.)