dbi-table: No special handling for data.table::IDate

[detule]

Closes #955

• Special handling no longer needed post date/time: handle r types with integer storage #954
• Removing, having it there makes writing less efficient

Tested with:

> library(data.table)
> datetime <- seq(as.POSIXct("2001-01-01"), as.POSIXct("2001-01-03"), by = "5 hour")
> tbl <- data.table(idate = as.IDate(datetime), a = rep(1:2, 5))
> .Internal(inspect(tbl$idate))
@55a4ffa26a38 13 INTSXP g0c4 [OBJ,REF(6),ATT] (len=10, tl=0) 11323,11323,11323,11323,11323,...
ATTRIB:
...<snip>...>
> dbWriteTable(conn, '#intsxp_test', tbl)
> DBI::dbReadTable(conn, '#intsxp_test')
          idate a
  1  2001-01-01 1
  2  2001-01-01 2
  3  2001-01-01 1
  4  2001-01-01 2
  5  2001-01-01 1
  6  2001-01-02 2
  7  2001-01-02 1
  8  2001-01-02 2
  9  2001-01-02 1
  10 2001-01-02 2
>

[simonpcouch]

Nice!