Remove register keyword which is deprecated in C++17

[Biswa96]

This fixes warnings with clang like this warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]

According to https://en.cppreference.com/w/cpp/keyword/register the keyword is unused and reserved.

Fixes https://github.com/jackaudio/jack2/issues/774