[API Compat] Ensure strict mode finds override removals

[ericstj]

Today we don't complain if an override is removed, even in strict mode, but we should. Removal of an override is binary compatible, but when folks are running in "strict" mode we should flag that. Overrides are relevant at compile time in non-sealed types since the compiler uses them when emitting calls to base.member.

sdk/src/Compatibility/Microsoft.DotNet.ApiCompatibility/Rules/MembersMustExist.cs

Lines 85 to 87 in 107c440

	// If method is an override or is promoted to the base type should not be reported.
	if (method.IsOverride || FindMatchingOnBaseType(method, containingType))
	return false;

See dotnet/arcade#2570

Repro: lib.zip