sql: always erase numeric flag after stringifying

Function which converts values to string representation
(sqlVdbeMemStringify()) erase MEM_Int/MEM_Real/MEM_Bool flags only when
it is specified by 'force' parameter. Hence, when 'force' argument is
false, memory cell after conversion will contain string value, but flag
indicating its type will be equal to combination of MEM_Str and one of
mentioned flags. It seems to be remains of affinity routines, since in
current state memory cell must have only one type.  What is more, it can
lead to unpredicted consequences, for instance assertion fault
(sql_value_type() assumes that value has one specific type). Let's fix
it removing 'force' argument from sqlVdbeMemStringify() and always clean
up type flag.