src/*: Remove VIM - Vi improved ... header

Regarding the individual items in the header:

`Vim - Vi improved	by Bram Moolenar`
  Bram Moolenar is already mentioned throughout the documentation, as
  well as the intro screen.

`:help uganda`
  It's already shown to all users who don't use `shortmess+=I` upon
  starting nvim, and is already placed prominently in help.txt, i.e.,
  `:help` run with no arguments.

`:help credits`
  Already mentioned near the top of help.txt.

`README.md`
  Already mentioned in develop.txt.

src/nvim/main.c

@@ -1,11 +1,3 @@
-/*
- * VIM - Vi IMproved	by Bram Moolenaar
- *
- * Do ":help uganda"  in Vim to read copying and usage conditions.
- * Do ":help credits" in Vim to see a list of people who contributed.
- * See README.md for an overview of the Vim source code.
- */
-
 #define EXTERN
 #include <assert.h>
 #include <stdint.h>
@@ -1864,5 +1856,3 @@ static void check_swap_exists_action(void)
     getout(1);
   handle_swap_exists(NULL);
 }
-
-