Do not deactivate gitgutter when &diff is set.

In #303 there was a request to deactivate gitgutter when &diff is set,
although no reason was given initially.  Then it was reported that
gitgutter showed signs, in the wrong place, during Fugitive's Gdiff.

However I cannot reproduce signs showing in Gdiff's index buffer.  And
the signs in the working tree buffer are correct.

Furthermore the commit which deactivated gitgutter (85f6e136) had the
side-effect of breaking `:redir` and clearing Vim's startup message
(see #500).

Therefore since that change had no benefit, and a couple of drawbacks,
it makes sense to undo it.

Closes #500.

autoload/gitgutter.vim

@@ -45,8 +45,6 @@ function! gitgutter#process_buffer(bufnr, force) abort
       endif
 
     endif
-  else
-    call s:clear(a:bufnr)
   endif
 endfunction