remove tests for unicode chars in regular expression literals.

mruby does not parse escape characters in a regular expression
literal (ref #2007).  It is parsed by the external mrbgem that
provides Regexp class, if any.  These tests should not be in
mruby core but in the mrbgem.
Merge pull request #2020 from iij/pr-remove-unicode-escape-tests

remove tests for unicode chars in regular expression literals.