patch 9.1.0899: default for 'backspace' can be set in C code

Problem:  default for 'backspace' can be set in C code
Solution: promote the default for 'backspace' from defaults.vim to the C
          code (Luca Saccarola)

closes: #16143

Signed-off-by: Luca Saccarola <github.e41mv@aleeas.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/defaults.vim

@@ -1,7 +1,7 @@
 " The default vimrc file.
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2024 Nov 14
+" Last Change:	2024 Dec 01
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 "
 " This is loaded if no vimrc file was found.
@@ -33,9 +33,6 @@ silent! while 0
   set nocompatible
 silent! endwhile
 
-" Allow backspacing over everything in insert mode.
-set backspace=indent,eol,start
-
 set ruler		" show the cursor position all the time
 set showcmd		" display incomplete commands
 

runtime/doc/options.txt

@@ -1069,8 +1069,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	done with ":syntax on".
 
 							*'backspace'* *'bs'*
-'backspace' 'bs'	string	(default "", set to "indent,eol,start"
-							    in |defaults.vim|)
+'backspace' 'bs'	string	(Vim default: "indent,eol,start", Vi default: "")
 			global
 	Influences the working of <BS>, <Del>, CTRL-W and CTRL-U in Insert
 	mode.  This is a list of items, separated by commas.  Each item allows

runtime/doc/version9.txt

@@ -41611,6 +41611,8 @@ Changed~
 - 'rulerformat' now supports the |stl-%!| item
 - the default 'history' option value has been increased to 200 and removed
   from |defaults.vim|
+- the default 'backspace' option for Vim has been set to "indent,eol,start"
+  and removed from |defaults.vim|
 
 							*added-9.2*
 Added ~