Revert "Twitter Timeline: add a message to explain 0 tweet-limit"

This reverts commit 2eedb2e8ed092f58f07f65daaaa92967a11bc776.
Twitter Timline: use null as default value for # of tweets
Null is the default value (unlimited amount of tweets)
Input now uses `number` type with `min` and `max` values as recommended in the twitter documentation:
https://dev.twitter.com/docs/embedded-timelines

See discussion in #6 for more details.