[Static Runtime] Remove dead code in MemoryPlanner and rename unmanaged_value_set to unmanaged_ivalue_set
Test Plan:
```
buck test mode/opt //caffe2/caffe2/fb/predictor:ptvsc2_predictor_bench_test -- --run-disabled
buck test //caffe2/benchmarks/static_runtime:static_runtime_cpptest
```
Reviewed By: bwasti
Differential Revision: D26827700
fbshipit-source-id: a8696af3e1d2b504fa5754f823b389d45b48af38
