Removed test issue-912.rs

This was marked xfail-test
According to rust's issue #912, this will not be fixed.
There's no point in keeping the test if it is never intended to pass.
Remove run-pass/issue-3907-2.rs

This test is not supposed to compile; there is an equivalent test in compile-fail/issue-3907.rs.
Remove test issue-3461

According to issue #3461, it doesn't sound like this is planned to be fixed.
auto merge of #10425 : astrieanna/rust/rm-issue-912, r=brson

According to rust's issue #912, this will not be fixed. There's no point in keeping the test if it is never intended to pass.