bdld_datum: Fix GCC unused variable error
bdld_datum: Fix GCC unused variable error

When building with `-Wall -Werror` and `BSLS_ASSERT_LEVEL_NONE`, the
`convertLength` value produces an error because it is unused.  There is
already a prior `BSLMF_ASSERT` verifying the constant value is the
number of elements computed via `sizeof()`, so just use it.

che2/bde,che2/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,che2/bde