[lld-macho][NFC] Refactor ObjCSelRefsSection => ObjCSelRefsHelper (#86456)

In a previous PR: https://github.com/llvm/llvm-project/pull/83878, the
intent was to make no functional changes, just refactor out the code for
reuse.
However, by creating `ObjCSelRefsSection` as a `SyntheticSection` - this
slightly changed the functionality of the application as the
`SyntheticSection` constructor registers the `SyntheticSection` as a
functional one - with an associated `SyntheticInputSection`.

With this change we remove this unintended consequence by making the
code not use a `SyntheticSection` as base, but just by having it be a
static helper.