Revert "eval: Implement a short path for pipelines containing one form."
Revert "eval: Implement a short path for pipelines containing one form."

This reverts commit a691ee911276b343b90fb81703b7f1081cd6fc38.

For reason not understood, this optimization causes performance regression on
the following code:

for i [(range 1000)] {
  for j [(range 1000)] {
  }
}

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish