test/ruby/test_io.rb: remove unnecessary begin/end

Unnecessary since r47422 when the "rescue IOError" clause
was removed.

* test/ruby/test_io.rb (test_readpartial_locktmp):
  remove unnecessary begin/end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48455 b2dd03c8-39d4-4d8f-98ff-823fe69b080e