update README: remove mentioning the old dextra repo

Thanks Christopher Drelich <cd@cdrakka.com>

README

@@ -18,9 +18,6 @@ necessary as root):
 
     make clean install
 
-If you are going to use the default bluegray color scheme it is highly
-recommended to also install the bluegray files shipped in the dextra package.
-
 
 Running dwm
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