dev: remove variable on Felt252Dict get()

[augustin-v]

The intermediate variable return_value in the dict get() method is redundant since the value implements Copy
and can be used both in finalize and return statement without being moved.

Removed an unused variable in the tests as well

[reviewable-StarkWare]

This change is 

[orizi]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @augustin-v)

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a discussion (no related file):
@gilbens-starkware for 2nd eye.

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corelib/src/test/dict_test.cairo line 114 at r1 (raw file):

    dict.insert(11, nullable::nullable_from_box(BoxTrait::new(2)));
    let val10 = dict[10].deref();
    let val11 = dict[11].deref();

revert.

[orizi]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @augustin-v)

[augustin-v]

@orizi Thank you for the review

[gilbens-starkware]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @augustin-v)

[orizi]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @augustin-v)