Fixing the redundency

[OmerFarukBOUN]

Describe the feature

zenoh-cpp/examples/universal/z_sub.cxx

Line 44 in b5fca3e

Config config = parse_args(argc, argv, {}, {{"key_expression", &expr}});

In this line the {"key_expression", &expr} part is redundant. In getargs.h, it parses this input but does nothing with it and this argument does not affect the "config" object. Since it is in the example codes, it makes the reader to believe it is necessary. There are more places that this was used.
Please delete these redundant parts.

[DenisBiryukov91]

This part is not redundant, it is used to parse optional key_expression argument value into &expr.