[feature] Blocks: remove identity else (else error -> error end) from with clauses

[novaugust]

Saw this while going thru existing codebase and was surprised we aren't already auto-rewriting it.
given a with statement with a valid multi head but an else clause that does nothing, remove the else clause

# input
with ...a do
  ...b
else
  error -> error
end

# output
with ...a do
  ....b
end

should be decently easy to implement with the existing nodes_equivalent? helper in blocks