subsys/debug: eliminate PEFORMANCE_METRICS and CPU_CLOCK_FREQ_MHZ

Neither of these options is actually used; CPU_CLOCK_FREQ_MHZ appears
to have been part of x86 BOOT_TIME_MEASUREMENT at some point, and
PERFORMANCE_METRICS is the stillborn cousin of EXECUTION_BENCHMARKS.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>