[NFC] Simplify a bit of logic in RemoveUnusedBrs

[kripken]

The checks in the old code were not needed: we have a block that
has a single child, another block. Any branch to the child sends a
value that flows out to the parent immediately. No circumstances
exist in which we can error.

(block $outer
  (block $inner
    ..code and a br to $inner, which can branch to $outer instead..
  )
)

Also, this code only handles blocks without a value, so even
subtyping is not an issue here. (Merging blocks with different
types is handled elsewhere, RemoveUnusedNames, so no
need to add new logic here.)

Background:

https://github.com/WebAssembly/binaryen/pull/8398/changes/BASE..82441de8882abbac2a83bd88596c985d8c093b8e#r2861953106

[tlively]

LGTM. Is it possible to find a test where would optimize now but not optimize before? Where the inner block result is a subtype of the outer block result, perhaps?

[kripken]

No, see this code only handles blocks without a value - this does not even run on blocks with results (another pass does those), so this is truly NFC.