Remove reference to GREP_COLORS from documentation

There is no longer support for external grep, as per bbc09c2 (grep: rip
out support for external grep, 2010-01-12), so remove the reference to it
from the documentation.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -680,9 +680,7 @@ color.grep::
 
 color.grep.match::
 	Use customized color for matches.  The value of this variable
-	may be specified as in color.branch.<slot>.  It is passed using
-	the environment variables 'GREP_COLOR' and 'GREP_COLORS' when
-	calling an external 'grep'.
+	may be specified as in color.branch.<slot>.
 
 color.interactive::
 	When set to `always`, always use colors for interactive prompts