selftests/resctrl: Remove unused measurement code

The MBM and MBA resctrl selftests run a benchmark during which
it takes measurements of read memory bandwidth via perf.
Code exists to support measurements of write memory bandwidth
but there exists no path with which this code can execute.

While code exists for write memory bandwidth measurement
there has not yet been a use case for it. Remove this unused code.
Rename relevant functions to include "read" so that it is clear
that it relates only to memory bandwidth reads, while renaming
the functions also add consistency by changing the "membw"
instances to more prevalent "mem_bw".

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>