Remove 'eshell-escape-arg'

Remove 'eshell-escape-arg'

Eshell no longer needs this function, since all command parsing is
performed first, with special syntax annotated with text properties as
needed, as opposed to marking literal text with a property (bug#54486).

* lisp/eshell/em-pred.el (eshell-modifier-alist): Make "q" modifier
obsolete.
(eshell-modifier-help-string): Remove mention of "q".

* lisp/eshell/esh-arg.el (eshell-escape-arg): Make obsolete.
(eshell-parse-backslash, eshell-parse-literal-quote)
(eshell-parse-double-quote): Don't call 'eshell-escape-arg'.

* lisp/eshell/esh-var.el (eshell-parse-variable): Don't call
'eshell-escape-arg'.

* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-test-2)
(em-extpipe-test-9, em-extpipe-test-11): Remove 'eshell-escape-arg'.

* test/lisp/eshell/em-pred-tests.el (em-pred-test/modifier-quote):
Remove test.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/quoted-interp-var-indices)
(esh-var-test/quote-interp-var-indices-subcommand): Remove workaround in
tests.

* doc/misc/eshell.texi (Argument Modifiers): Remove documentation of
obsolete "q" modifier.

(cherry picked from commit b4655ff99b512f30220f22226514267d78a70605)