[llvm-mca] Remove the logic that computes the reciprocal throughput, and make...

[llvm-mca] Remove the logic that computes the reciprocal throughput, and make the SummaryView independent from the Backend. NFCI

Since r327420, the tool can query the MCSchedModel interface to obtain the
reciprocal throughput information.
As a consequence, method `ResourceManager::getRThroughput`, and
method `Backend::getRThroughput` are no longer needed.

This patch simplifies the code by removing the custom RThroughput computation.
This patch also refactors class SummaryView by removing the dependency with
the Backend object.

No functional change intended.

llvm-svn: 327425