rcfile, docs: no longer recognize nor mention 'set backwards'

The option has been a no-op since 2.9.0.  And now that the
^W/^Q/M-W/M-Q set is available, 'set backwards' is entirely
outdated.

doc/nano.texi

@@ -718,10 +718,6 @@ numbered one every time a file is saved --- when backups are enabled
 with @code{set backup} or @option{--backup} or @option{-B}.
 The uniquely numbered files are stored in the specified directory.
 
-@item set backwards
-Obsolete option.  Recognized but ignored.  @code{^Q} is available to
-start a backward search.
-
 @item set boldtext
 Use bold instead of reverse video for the title bar, status bar, key combos,
 function tags, line numbers, and selected text.  This can be overridden by

doc/nanorc.5

@@ -80,10 +80,6 @@ numbered one every time a file is saved -- when backups are enabled
 with \fBset backup\fR or \fB\-\-backup\fR or \fB\-B\fR.
 The uniquely numbered files are stored in the specified \fIdirectory\fR.
 .TP
-.B set backwards
-Obsolete option.  Recognized but ignored.  \fB^Q\fR is available to
-start a backward search.
-.TP
 .B set boldtext
 Use bold instead of reverse video for the title bar, status bar, key combos,
 function tags, line numbers, and selected text.  This can be overridden by

src/rcfile.c

@@ -94,7 +94,6 @@ static const rcoption rcopts[] = {
 	{"autoindent", AUTOINDENT},
 	{"backup", BACKUP_FILE},
 	{"backupdir", 0},
-	{"backwards", BACKWARDS_SEARCH},
 	{"casesensitive", CASE_SENSITIVE},
 	{"cut", CUT_FROM_CURSOR},  /* deprecated form, remove end of 2018 */
 	{"cutfromcursor", CUT_FROM_CURSOR},