Remove special type config in inputs/base

As the special filtering on type is deprecated in filters and outputs, having a special configuration to set the type in the input does not really make sense.

The issue is that the current logic is not equivalent to the add_field behaviour

```
# If you try to set a type on an event that already has one (for
# example when you send an event from a shipper to an indexer) then
# a new input will not override the existing type. A type set at
# the shipper stays with that event for its life even
# when sent to another Logstash server.
```

Meaning that a more generic config **add_field_if_not_exist** should be introduced and a cooler name should be find, thoughts ?