Do not adjust SignColumn highlight

This was introduced in 07d7c9d4 because many colorschemes at the time
had ugly sign columns, and people kept asking how to deal with it.

These days there are many more plugins which use the sign column and
consequently more colorschemes pay attention to the SignColumn highlight
group.

Therefore it no longer makes sense for vim-gitgutter to adjust the
sign column's appearance.

To restore the previous behaviour, add this to your vimrc:

    highlight! link SignColumn LineNr

See #696.

doc/gitgutter.txt

@@ -306,7 +306,6 @@ Signs:~
     |g:gitgutter_sign_removed|
     |g:gitgutter_sign_removed_first_line|
     |g:gitgutter_sign_modified_removed|
-    |g:gitgutter_override_sign_column_highlight|
 
 Hunk previews:~
 
@@ -450,20 +449,6 @@ Defaults:
 You can use unicode characters but not images.  Signs must not take up more than
 2 columns.
 
-                                   *g:gitgutter_override_sign_column_highlight*
-Default: 1
-
-Controls whether to make the sign column look like the line-number column (i.e.
-the |hl-LineNr| highlight group).
-
-To customise your sign column's background color, first tell vim-gitgutter to
-leave it alone:
->
-    let g:gitgutter_override_sign_column_highlight = 0
-<
-
-And then either update your colorscheme's |hlSignColumn| highlight group or set
-it in your |vimrc|:
 
   Desired appearance                  Command ~
   Same as line-number column          highlight clear SignColumn
@@ -596,8 +581,11 @@ c. Why can't I unstage staged changes?
 d. Why are the colours in the sign column weird?
 
   Your colorscheme is configuring the |hl-SignColumn| highlight group weirdly.
-  Please see |g:gitgutter_override_sign_column_highlight| on customising the
-  sign column.
+  Here are two ways you could change the colours:
+>
+    highlight! link SignColumn LineNr
+    highlight SignColumn guibg=whatever ctermbg=whatever
+<
 
 e. What happens if I also use another plugin which uses signs (e.g. Syntastic)?