Deprecate Throwables.propagate with RuntimeException in ManuallyScheduleHeartbeat.java

[yupeng9]

Guava "Throwables.propagate" is deprecated. see reasons in
https://github.com/google/guava/wiki/Why-we-deprecated-Throwables.propagate

This ticket modifies https://github.com/Alluxio/alluxio/blob/master/core/common/src/test/java/alluxio/heartbeat/ManuallyScheduleHeartbeat.java#L65 to replace

throw Throwables.propagate(e);

or

Throwables.propagate(e);

with

throw new RuntimeException(e);

Remember also to remove the corresponding import if Throwables becomes unused

import com.google.common.base.Throwables;