storage, db, mixer, command: remove more bogus "pure" attributes

This commit is similar to 788e3b31e1,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

src/command/Request.hxx

@@ -45,68 +45,57 @@ public:
 			     : default_value;
 	}
 
-	gcc_pure
 	int ParseInt(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgInt(data[idx]);
 	}
 
-	gcc_pure
 	int ParseInt(unsigned idx, int min_value, int max_value) const {
 		assert(idx < size);
 		return ParseCommandArgInt(data[idx], min_value, max_value);
 	}
 
-	gcc_pure
 	int ParseUnsigned(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgUnsigned(data[idx]);
 	}
 
-	gcc_pure
 	int ParseUnsigned(unsigned idx, unsigned max_value) const {
 		assert(idx < size);
 		return ParseCommandArgUnsigned(data[idx], max_value);
 	}
 
-	gcc_pure
 	bool ParseBool(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgBool(data[idx]);
 	}
 
-	gcc_pure
 	RangeArg ParseRange(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgRange(data[idx]);
 	}
 
-	gcc_pure
 	float ParseFloat(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgFloat(data[idx]);
 	}
 
-	gcc_pure
 	SongTime ParseSongTime(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgSongTime(data[idx]);
 	}
 
-	gcc_pure
 	SignedSongTime ParseSignedSongTime(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgSignedSongTime(data[idx]);
 	}
 
-	gcc_pure
 	int ParseOptional(unsigned idx, int default_value) const {
 		return idx < size
 			? ParseInt(idx)
 			: default_value;
 	}
 
-	gcc_pure
 	RangeArg ParseOptional(unsigned idx, RangeArg default_value) const {
 		return idx < size
 			? ParseRange(idx)