Finish Removing Nonmutating

[InsertCreativityHere]

This PR finishes removing nonmutating.
This is a sister PR to #2121, which removed the last of the 'nonmutating' metadata checks.
As-of this PR, we should no longer be sending nonmutating, nor making any distinction between it and idempotent.

This PR:

• Changes the built-in Object operations (ice_isA, ice_ping, ice_id, ice_ids) to use OperationMode::Idempotent, instead of OperationMode::Nonmutating which they're currently using.
• Deletes the parts of the 'Ice/operations' test which were solely testing which operation mode those operations used.
• Updates the Slice doc comments for OperationMode

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In Ruby, I couldn't find how/where we specify the operation mode these 4 built-ins use...
We define the operations here (v) but I couldn't figure out how an operation mode gets specified.

ice/ruby/src/IceRuby/Proxy.cpp

Lines 1214 to 1217 in 6002b49

	rb_define_method(_proxyClass, "ice_isA", CAST_METHOD(IceRuby_ObjectPrx_ice_isA), -1);
	rb_define_method(_proxyClass, "ice_ping", CAST_METHOD(IceRuby_ObjectPrx_ice_ping), -1);
	rb_define_method(_proxyClass, "ice_ids", CAST_METHOD(IceRuby_ObjectPrx_ice_ids), -1);
	rb_define_method(_proxyClass, "ice_id", CAST_METHOD(IceRuby_ObjectPrx_ice_id), -1);