Remove mutex debugging code

Remove mutex debugging code

Mutex debugging code (used when the ISC_MUTEX_DEBUG preprocessor macro
is set to 1 and PTHREAD_MUTEX_ERRORCHECK is defined) has been broken for
the past 3 years (since commit 2f3eee5a4fdad6606135116c70875b3180c7ed83)
and nobody complained, which is a strong indication that this code is
not being used these days any more.  External tools for detecting
locking issues are already wired into various GitLab CI checks.  Drop
all code depending on the ISC_MUTEX_DEBUG preprocessor macro being set.