Fix RBMap's, iterator-based, remove()

[RandomShaper]

RBMap::remove() needs to access private members of its Iterator, but there's no friendship between them, so it's broken.

This PR is here to save the day.

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Test code:

	RBMap<int, int> rb_map;
	decltype(rb_map)::Iterator rb_it;
	rb_map.remove(rb_it);

MSVC error:

core/templates/rb_map.h(166): error C2248: 'RBMap<int,int,Comparator<K>,DefaultAllocator>::Iterator::E': cannot access private member declared in class 'RBMap<int,int,Comparator<K>,DefaultAllocator>::Iterator'

[akien-mga]

Thanks!