Remove mutex profiling code
Mutex profiling code (used when the ISC_MUTEX_PROFILE preprocessor macro is set to 1) has been broken for the past 3 years (since commit 0bed9bfc) and nobody complained, which is a strong indication that this code is not being used these days any more. External tools for both measuring performance and detecting locking issues are already wired into various GitLab CI checks. Drop all code depending on the ISC_MUTEX_PROFILE preprocessor macro being set.