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

[simonwallis2]

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

llvm/test/CodeGen/AVR/return.ll

 ; CHECK: ldd r24, Y+5
 ; CHECK: ldd r25, Y+6
 ; CHECK: pop r29
 ; CHECK: pop r28
   %result = trunc i64 %d to i32
   ret i32 %result
 }
 
-define i32 @naked(i32 %x) naked {
-; CHECK-LABEL: naked:
-; CHECK-NOT: ret
-  ret i32 %x
-}
-
 define avr_intrcc void @interrupt_handler() {
 ; CHECK-LABEL: interrupt_handler:
 ; CHECK: reti
   ret void
 }
 
 define avr_signalcc void @signal_handler() {
 ; CHECK-LABEL: signal_handler:
 ; CHECK: reti
   ret void
 }