Avoid backslash-newline-newline in source code

Avoid backslash-newline-newline in source code

* etc/refcards/Makefile (PDF_FRENCH):
* lib-src/etags.c (LOOP_ON_INPUT_LINES):
* lisp/dabbrev.el (dabbrev-check-other-buffers):
* lisp/org/org-id.el (org-id-link-to-org-use-id):
* lisp/org/org.el (org-support-shift-select, org-file-apps):
* src/alloc.c (CHECK_ALLOCATED_AND_LIVE)
(CHECK_ALLOCATED_AND_LIVE_SYMBOL):
* src/frame.h (FRAME_PIXEL_WIDTH_TO_TEXT_COLS):
* src/regex-emacs.c (PREFETCH_NOLIMIT):
* src/window.h (WINDOW_BUFFER):
Remove backslash-newline that immediately precedes another
newline, as this is not the usual style and is confusing.