lib/benchmark.rb: remove CLOCK_MONOTONIC_RAW support

In addition to being unaffected by _offset_ correction,
CLOCK_MONOTONIC_RAW is also unaffected by _frequency_ correction,
making it unsuitable for measuring real time on systems where the
clock is always running too fast or slow.

CLOCK_MONOTONIC (without _RAW) is the correct clock, as it is unaffected
by _offset_ correction (due to human error or battery replacement), but
still takes _frequency_ correction into account for clocks which
consistently run too fast or slow.

Thanks to Vít Ondruch for reporting the issue on ARM [Bug #10202]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47526 b2dd03c8-39d4-4d8f-98ff-823fe69b080e