vim-patch:9.1.0899: default for 'backspace' can be set in C code (#31416)

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: vim/vim#16143

959ef61430

N/A patches:
vim-patch:9.1.0895: default history value is too small
vim-patch:075aeea: runtime(doc): document changed default value for 'history'

Co-authored-by: Luca Saccarola <github.e41mv@aleeas.com>

test/old/testdir/setup.vim

@@ -1,6 +1,5 @@
 if exists('s:did_load')
   " Align Nvim defaults to Vim.
-  set backspace=
   set commentstring=/*\ %s\ */
   set complete=.,w,b,u,t,i
   set define=^\\s*#\\s*define