Revert Tramp commit from 2017-11-20

* lisp/net/tramp.el (tramp-drop-volume-letter)
(tramp-handle-find-backup-file-name): Revert change from
2017-11-20.  Emacs' `make-backup-file-name-1' handles this
case now.

lisp/net/tramp.el

@@ -1858,8 +1858,7 @@ letter into the file name.  This function removes it."
      (if (tramp-compat-file-name-quoted-p name)
 	 'tramp-compat-file-name-quote 'identity)
      (let ((name (tramp-compat-file-name-unquote name)))
-       ;; A volume letter could occur also in encoded backup file names.
-       (if (string-match "\\(\\`[[:alpha:]]:/\\|/!drive_[[:alpha:]]\\)" name)
+       (if (string-match "\\`[a-zA-Z]:/" name)
 	   (replace-match "/" nil t name)
 	 name)))))
 
@@ -3226,23 +3225,21 @@ User is always nil."
 (defun tramp-handle-find-backup-file-name (filename)
   "Like `find-backup-file-name' for Tramp files."
   (with-parsed-tramp-file-name filename nil
-    (mapcar
-     'tramp-drop-volume-letter
-     (let ((backup-directory-alist
-	    (if tramp-backup-directory-alist
-		(mapcar
-		 (lambda (x)
-		   (cons
-		    (car x)
-		    (if (and (stringp (cdr x))
-			     (file-name-absolute-p (cdr x))
-			     (not (tramp-tramp-file-p (cdr x))))
-			(tramp-make-tramp-file-name
-			 method user domain host port (cdr x) hop)
-		      (cdr x))))
-		 tramp-backup-directory-alist)
-	      backup-directory-alist)))
-       (tramp-run-real-handler 'find-backup-file-name (list filename))))))
+    (let ((backup-directory-alist
+	   (if tramp-backup-directory-alist
+	       (mapcar
+		(lambda (x)
+		  (cons
+		   (car x)
+		   (if (and (stringp (cdr x))
+			    (file-name-absolute-p (cdr x))
+			    (not (tramp-tramp-file-p (cdr x))))
+		       (tramp-make-tramp-file-name
+			method user domain host port (cdr x) hop)
+		     (cdr x))))
+		tramp-backup-directory-alist)
+	     backup-directory-alist)))
+      (tramp-run-real-handler 'find-backup-file-name (list filename)))))
 
 (defun tramp-handle-insert-directory
   (filename switches &optional wildcard full-directory-p)