patch 9.1.0609: outdated comments in Makefile

patch 9.1.0609: outdated comments in Makefile

Problem:  outdated comments in Makefile
Solution: update outdated comments, update rule for vimtags
          (Shane Harper)

related: commit b81109192f

Here are the changes and the reasons for them:

- Delete the comment preceding the assignment to VIMPROG. Since b81109192f
  there's no need for VIMPROG to be set to something else when this is executed
  from src/Makefile. (The comment was wrong anyway; VIMPROG was being set to
  "$$BUILD_DIR/$(VIMTARGET)".)

```
    # Set to $(VIMTARGET) when executed from src/Makefile.
     VIMPROG = ../../src/vim
```

- Delete "`and installed`" in the following comment; The vimtags rule doesn't
  require that Vim has been installed.

```
    # Use Vim to generate the tags file.  Can only be used when Vim has been
    # compiled and installed.  Supports multiple languages.
    vimtags: $(DOCS)
```

- With commit b81109192f there is no longer a need to set VIMPROG here:

```
       -@BUILD_DIR="`pwd`"; cd $(HELPSOURCE); if test -z "$(CROSS_COMPILING)"; then \
               $(MAKE) VIMPROG="$$BUILD_DIR/$(VIMTARGET)" vimtags; fi
```

The new code below will use the same vim executable as the old code:
```
       -@cd $(HELPSOURCE); if test -z "$(CROSS_COMPILING)"; then \
               $(MAKE) vimtags; fi
```

- Delete the following comment which was related to setting VIMPROG as it no longer has any value:
```
       # We can assume Vim was build, but it may not have been installed,
       # thus use the executable in the current directory.
```
Note: this comment used to be (unnecessarily) echoed to the terminal (because
      it was indented) when making installrtbase.

closes: #15320

Signed-off-by: Shane Harper <shane@shaneharper.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>