maint: Remove `#if 0` commented-out code from the codebase. The codebase had 1850+ lines of code that were commented out using the `#if 0` preprocessor directive. Running `hg annotate` indicated that most of them had been commented out several years ago and had not been edited in years, except maybe whitespace formatting, leading to a large amount of commented-out code that was not up-to-date with the rest of the codebase. This edit removes almost all instances of `#if 0` from .h and .cpp files, the only exception being a corner case where /* */ comments cannot be used as they will interfere with BIST formatting. If this removal proves to be too disruptive, please revert as required.