Mark the float8 -> int8 cast as implicit. This resolves the problem

Mark the float8 -> int8 cast as implicit.  This resolves the problem
pointed out by Barry Lind: UPDATE bigintcol = 10000000000 fails because
the constant is initially taken as float8.  We really need a better way,
but it's not gonna happen for 7.3.

Also, remove int4reltime() function, which is redundant with the
existing binary-compatibility coercion path from int4 to reltime,
and probably has been unreachable code for a long while.