Remove declaration causing an unused variable warning on gcc 5.1 (bug #23601)

It seems to have been meant to silence such a warning on some other
compiler/version, but this might not be necessary anymore since commit
07b6eeedb2 adds the unused attribute to
some of the symbols generated by the MAKE_NUM macro.

At least I don't get a warning with gcc 4.9 or clang 3.4 this way.

src/unit_filter.cpp

@@ -96,8 +96,6 @@ namespace conditional {
 		(NOT, "not")
 	)
 	MAKE_ENUM_STREAM_OPS1(TYPE)
-
-	static TYPE warning_suppressor = string_to_TYPE_default("foo", NOT);
 }
 
 /// This class lazily parses an attribute value to a vector of strings