Remove feature gitgutter_escape_grep

Reverts feature introduced in 5c23cadf57

In order to use an escaped grep, please replace
`g:gitgutter_escape_grep=1` with:

    let g:gitgutter_grep_command = '\grep --color=never -e'

plugin/gitgutter.vim

@@ -46,7 +46,6 @@ endtry
 
 call s:set('g:gitgutter_sign_modified_removed',    '~_')
 call s:set('g:gitgutter_diff_args',                  '')
-call s:set('g:gitgutter_escape_grep',                 0)
 call s:set('g:gitgutter_map_keys',                    1)
 call s:set('g:gitgutter_avoid_cmd_prompt_on_windows', 1)