simpleperf: Remove stack and reg fields in samples after unwinding

perf_data_converter doesn't support stack and regs fields.
This is to create recording file parseable by perf_data_converter.

Bug: 273341791
Test: run simpleperf_unit_test
Change-Id: Ie05c683eb2f3eec3e08e482b82cbc0be4abe834a
6 files changed