tests: boot_time: remove all the code and instrumentation feeding into test

Remove the config BOOT_TIME_MEASUREMENT and corresponding #ifdef'd code
throughout (kernel/init.c, idle.c, core/common.S , reset.S, ... ) which
hold the extern hooks for z_timestamp_main and z_timestamp_idle in the
removed boot_time test suite.

Signed-off-by: Jennifer Williams <jennifer.m.williams@intel.com>