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/if_cscope.c

@@ -4,9 +4,8 @@
  *
  * The basic idea/structure of cscope for Vim was borrowed from Nvi.  There
  * might be a few lines of code that look similar to what Nvi has.
- *
- * See README.md for an overview of the Vim source code.
  */
+
 #include <stdbool.h>
 
 #include <assert.h>