Consider removing the `register` keyword from C++ code

[irrenhaus3]

Seen in https://github.com/jackaudio/jack2/blob/1ff83b5314b257b2745b33dc9bd76a96b45cdc8b/common/JackAudioPort.cpp#L67

register is a reserved and unused keyword since C++17 as described here https://en.cppreference.com/w/cpp/keyword/register, and has been effectively no-op for a long time. To be future-proof, consider removing the register keyword from the codebase, since it will no longer compile in any compliance mode from C++17 onwards.