Bugfix https://tizendev.org/bugs/browse/TDIS-4756

Bugfix https://tizendev.org/bugs/browse/TDIS-4756

[Issue#] N/A
[Problem] Conversion of IRI encoded in Unicode to ASCII failed
on some platforms but on others succeeded.
[Cause] Due to char type being treated as signed on some platforms and
unsigned on others.
[Solution] The code that causes the issue is not used so it has been
removed.
[SCMRequest] N/A

[Verification]
1. Build respository.
2. Run `wrt-extra-tests-w3c-widget-policy --output=text
--regexp='load_image_using_toASCII'`

Change-Id: Ie56e2b6a3b673617350836d688032c63c0b9c0b1