This commit removes support for gprof-based profiling (the -p option to ocamlopt). It follows a discussion on the core developers' list, which indicated that removing gprof support was a reasonable thing to do. The rationale is that there are better easy-to-use profilers out there now, such as perf for Linux and Instruments on macOS; and the gprof support has always been patchy across targets. We save a whole build of the runtime and simplify some other parts of the codebase by removing it.