sql: don't change type of function's retval after codegen

Proper type of function's returning value is set during names resolution
(i.e. promotion from struct FuncDef to struct Expr, see
resolveExprStep()). Accidentally, it was set again during byte-code
generation for VDBE. What is more, in some cases it was set to a wrong
type. For instance, built-in function randomblob() returns value to be
encoded as MP_BIN, so its returning type is SCALAR. However, it was
reset to INTEGER (as its first argument). This patch simply removes this
second type promotion.