Remove (or double) redundant backslashes in string literals

Remove (or double) redundant backslashes in string literals

See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00749.html .

* lisp/obsolete/iswitchb.el (iswitchb-summaries-to-end):
* test/src/regex-emacs-tests.el (regex-tests-BOOST-frob-escapes):
* test/lisp/help-fns-tests.el (help-fns-test-lisp-macro)
(help-fns-test-lisp-defun, help-fns-test-lisp-defsubst)
(help-fns-test-alias-to-defun, help-fns-test-bug23887):
Double backslashes for desired effect.
* lisp/org/ol.el (org-link-escape):
* lisp/net/nsm.el (nsm-protocol-check--rsa-kx)
(nsm-protocol-check--anon-kx, nsm-protocol-check--sha1-sig):
* lisp/obsolete/old-whitespace.el (whitespace-buffer):
* lisp/obsolete/rcompile.el (remote-compile-run-before):
* lisp/obsolete/vi.el (vi-end-of-blank-delimited-word):
* lisp/obsolete/vip.el (vip-current-major-mode)
(vip-paren-match, vip-switch-to-buffer)
(vip-switch-to-buffer-other-window, vip-kill-buffer)
(vip-get-ex-token, ex-edit):
* lisp/org/org-element.el (org-element--cache-sync-requests):
* lisp/org/org.el (org-sparse-tree):
* lisp/textmodes/reftex.el (reftex-report-bug):
* test/lisp/ibuffer-tests.el (ibuffer-save-filters):
* test/lisp/international/ucs-normalize-tests.el
(ucs-normalize-tests--insert-failing-lines):
* test/lisp/simple-tests.el (undo-test-kill-c-a-then-undo):
* test/lisp/textmodes/conf-mode-tests.el (conf-test-toml-mode):
* test/src/regex-emacs-tests.el (regex-tests-compare):
Remove redundant backslashes.