[lld-macho][nfc] Remove TODO regarding addends

There was initially some concern around the correct handling of pcrel
section relocations with r_length != 2. But it looks like there are no such
relocations in practice -- x86_64's pcrel section relocs all have r_length == 2,
and ARM64 doesn't even have pcrel section relocs. So we can replace the TODO
with an assert.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D97576