[AVR][test] Remove test for naked function containing a return.

Summary:
Remove an old test for an explicit return in a naked function from test/CodeGen/AVR/return.ll

clang no longer allows a C return in a naked function.

This test is causing failure of the patch https://reviews.llvm.org/D80715

Reviewers: llvm-commits, dylanmckay

Reviewed By: dylanmckay

Subscribers: Jim

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81099

Change-Id: Id218027e520247ae480b92e7801a660fbe0cf29b