Strip text property of inserted string

Some face properties are remained if font-lock-defaults is nil, such as
text-mode, fundamental-mode etc.


Reproduce steps are
1. Enable `auto-complete-mode` in text-mode
2. Call `auto-complete`
3. Call ac-isearch(`C-s`)
4. Decide candidate

Then remaining `popup-isearch-match` face property as below.

![auto-complete-remaining-face-property](https://f.cloud.github.com/assets/554281/1590061/1c6a62c8-5285-11e3-85ba-58770d88b3cd.png)