build: clear 54 dead private fields
54 of the 161 -Wunused-private-field warnings, across 31 files. These are
the ones needing no judgment. Each member is declared once and appears
nowhere else in src/, counting the .mm and .c sources as well as .cpp and
.hpp, so nothing writes them and nothing reads them. Every removal is a
whole line, and no declaration shares a line with another member.
The remaining 107 are left alone. Those members are mentioned elsewhere,
usually assigned and never read, where the fix might be deleting the
member or might be restoring a read that went missing.
