`std.container.dlist` supports `linearRemoveElement`

`linearRemoveElement` removes the first occurence of an element from the dlist

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import std.container : Dlist;
import std.algorithm.comparison : equal;

auto a = DList!int(-1, 1, 2, 1, 3, 4);
a.linearRemoveElement(1);
assert(equal(a[], [-1, 2, 1, 3, 4]));
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