Remove odd special case of some parse error recovery functions

`recover_from_inc_dec` *always* returns a (fatal) `Err(_)` *except* if
the increment/decrement operator is a subexpression *and* the source of
the operand is not available in which case it emits the diagnostic and
returns `Ok(_)` (rendering it non-fatal).

This makes no sense whatsoever. For illustration purposes, listed below
are steps that would make us reach this case:

1. `rustc a.rs --crate-type=lib` where `a.rs` contains:
   `#[macro_export] macro_rules! m { () => { i++ } }`.
2. Move or remove `a.rs`
3. `rustc b.rs --edition 2018 --extern a -L.` where
   `b.rs` contains:
   `fn main() { (a::m!()); }`.

Just make the error unconditionally fatal and add a FIXME to make it non
fatal in the future which would allow us to report name resolution errors
and what not. However, since that would be slightly more involved and
represent a behavior change (in the error path), this is out of scope for
a mere cleanup commit like this one.