[AVR] Don't adjust for instruction size

I'm not entirely sure why this was ever needed, but when I remove both
adjustments all tests still pass.

This fixes a bug where a long branch (using the `jmp` instead of the
`rjmp` instruction) was incorrectly adjusted by 2 because it jumps to an
absolute address instead of a PC-relative address. I could have added
AVR::fixup_call to the list of exceptions, but it seemed more sensible
to me to just remove this code.

Differential Revision: https://reviews.llvm.org/D78459