alternator: remove decimal_type FIXME
alternator: remove decimal_type FIXME

Decimal precision problems were already solved by commit
d5a1854d93c9448b1d22c2d02eb1c46a286c5404, but one FIXME
remained in the code by mistake.

Message-Id: <9ad4503a7b95ed851c6211d9b3615220e981adab@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla