remove useless const modifier

[JustAGod1]

The 'const' modifier in these functions serves no purpose. However, it will cause GCC or Clang to generate a warning in a file where rdkafka.h is included, which will result in a compilation failure if -Werror is enabled .

[cla-assistant[bot]]

All committers have signed the CLA.

[emasab]

/sem-approve

[wanders]

For searchability for others hitting the same error here is the complete error message generated by gcc:

librdkafka/rdkafka.h:8205:11: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]

clang says:

warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]

[JustAGod1]

Rebased master

[Quuxplusone]

Duplicates #4556 and #4504 (but yes, @emasab or someone, please merge either one of these already, and close the other!)