Simplify how Eshell's iterative evaluation handles 'if' forms

Simplify how Eshell's iterative evaluation handles 'if' forms

The previous implementation used 'eshell-test-body' and
'eshell-command-body' to track the condition and the then/else forms,
but those special variables are only needed for looping.  'if' only
evaluates each form once at most (bug#61954).

* lisp/eshell/esh-cmd.el (Command evaluation macros): Remove 'if' from
the notes about 'eshell-test-body' and 'eshell-command-body'.
(eshell-do-eval): Reimplement evaluation of 'if' forms.
(eshell-eval-command): Don't let-bind 'eshell-command-body' and
'eshell-test-body'; they're no longer needed here.