Revert "Handle sub-paths as relative paths when completing"

Although the tests for grepper#complete still pass after this change,
when completing from the prompt, each completion after this adds another
leading dot and slash, which suggests that maybe I need to testing
grepper#complete_files directly (prompt uses grepper#complete_files).

This reverts commit 5132e85.

plugin/grepper.vim

@@ -241,6 +241,9 @@ function! grepper#complete_files(lead, _line, _pos)
   " handle paths in $HOME (~/foo)
   elseif path[0:1] ==# '~/'
     return map(split(globpath($HOME, path[2:].'*'), '\n'), 'head . "~" . v:val['.len($HOME).':] . (isdirectory(v:val) ? s:slash : "")')
+  " handle sub paths
+  elseif path =~ '^.\/'
+    return map(split(globpath('.'.s:slash, path[2:].'*'), '\n'), 'head . "." . v:val[1:] . (isdirectory(v:val) ? s:slash : "")')
   " handle absolute paths
   elseif path[0] == '/'
     return map(split(globpath(s:slash, path.'*'), '\n'), 'head . v:val[1:] . (isdirectory(v:val) ? s:slash : "")')