.gdbinit: revert colored prompt

* .gdbinit: revert colored prompt because incremental search does not
  work well with invisible sequence in prompt.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39288 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.gdbinit

@@ -10,7 +10,7 @@ define ruby_gdb_init
   end
 end
 
-set prompt \033[36m(gdb)\033[m\040
+# set prompt \033[36m(gdb)\033[m\040
 
 define rp
   ruby_gdb_init