Stop checking for `tizen_mobile' in gyp wherever possible.

tizen_mobile has not been defined ever since we dropped support for
Tizen 2.x in 58841e7393074ea40e9a86bb55e8025256cf5af5, so the check has
been dead for a while.

Helps fix XWALK-1625.
Merge pull request #1901 from rakuco/tizen_mobile-cleanup

Stop checking for `tizen_mobile' in gyp wherever possible.