[AVR] Remove the select-mbb-placement-bug.ll test

This test was originally added when an old bug was fixed that caused
broken iterator code to break basic block placement.

The issue has an extremely low chance of every being a problem again.

This specific test is very flaky and fails often due to upstream
changes.

I have removed this test because it negates more value than it returns.

llvm-svn: 318134