[llvm-exegesis][NFC] remove runAndMeasure

This completes the FIXME listed in FunctionExecutor in regards to
deprecating this function. It simply makes the appropriate call into
runAndSample and grabs the first counter value. This patch completely
removes the function, moving that logic into the callers (currently only
uopsBenchmarkRunner). This makes creating new FunctionExecutors easier
as an implementation no longer needs to worry about this detail.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D147878