Remove core metrics

[ironage]

The internal core metrics haven't been used in a long time. I'm removing them now, and if they are needed in the future we can look at the git history.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• C-API, if public C++ API changed.

[coveralls-official]

Pull Request Test Coverage Report for Build james.stone_370

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 85 unchanged lines in 19 files lost coverage.
• Overall coverage increased (+0.003%) to 91.211%

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Files with Coverage Reduction	New Missed Lines	%
test/test_dictionary.cpp	1	99.85%
src/realm/object-store/sync/sync_manager.cpp	2	86.9%
src/realm/parser/generated/query_flex.cpp	2	68.61%
src/realm/query.cpp	2	93.6%
src/realm/sync/network/websocket.cpp	2	75.46%
src/realm/table_view.cpp	2	93.91%
src/realm/query_expression.hpp	3	93.51%
src/realm/sync/noinst/protocol_codec.hpp	3	76.56%
src/realm/sync/transform.cpp	3	61.71%
src/realm/util/file_mapper.hpp	3	93.18%

Totals
Change from base Build 1692:	0.003%
Covered Lines:	232434
Relevant Lines:	254832

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💛 - Coveralls

[michael-wb]

LGTM - it's always nice to remove unused code and our code coverage went up a tiny bit :)