Remove mutex profiling code

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
0bed9bfc28a204cde57c6f68170ecc89ebfa6dc8) 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.