This patch removes the FAST_MUTEX code and the related
WITH_FAST_MUTEXES CMake option. Note that this option
was enabled by default for Linux release builds.

Performance tests show that "fast" mutexes were equal to
or (more often) slower than default Linux mutexes.

Removing fast mutexes fixes / makes obsolete:
Bug#11748914: MYSQL PERFORMANCE WITH AND WITHOUT
              FAST MUTEXES USING SYSBENCH WORKLOAD
Bug#13923722: FAST MUTEXES DELAY SHOULD BE CONFIGURABLE -
              DEFAULT BAD ON SUPRA03/01
Bug#18870931: MUTEX_DELAY() CREATING EXCESS MEMORY TRAFFIC,
              GCC MEM BARRIER NEEDED
Bug#18871517: MUTEX_DELAY() MISSING X86 PAUSE INSTRUCTION
              OPTIMIZATION
Bug#18871138: SET THREAD PRIORITY IN MY_PTHREAD_FASTMUTEX_LOCK