Revert "reduce: handle empty updates (fix #1313)"

This reverts commit e24af3c78e.

While the semantics are desirable, there is no way to implement them
efficiently.  The reason is that in order to handle backtracking (empty)
from the state update expression, we have to retain a reference to the
reduction state value in order to restore it upon backtracking.
Retaining a reference to the reduction state kills performance by
causing lots of additional memory allocations and garbage because the
input to the update expression will always have at least two references,
thus no changes to it can be done in-place, and all changes end up being
CoW changes.

Avoiding this is the very reason for the LOADVN instruction (leaving
`null` in the variable loaded from).

docs/content/3.manual/manual.yml

@@ -2596,18 +2596,10 @@ sections:
                   (2 as $item | . + $item) |
                   (1 as $item | . + $item)
 
-          If the reduction update expression outputs `empty` (that is,
-          no values), then the reduction state is left as-is.  For
-          example, `reduce range(4) as $n ({}; if .==2 then empty else
-          .[$n|tostring] |= $n)` will produce `{"0":0,"1":1,"3":3}`.
-
         examples:
           - program: 'reduce .[] as $item (0; . + $item)'
             input: '[10,2,5,3]'
             output: ['20']
-          - program: 'reduce .[] as $n ([]; if $n%2==0 then empty else . + [$n] end)'
-            input: '[0,1,2,3,4,5]'
-            output: ['[1,3,5]']
 
       - title: "`isempty(exp)`"
         body: |