Fix: incorrect TypeMismatch when dealing with literals in lists

[iamdefinitelyahuman]

What I did

Fix an issue preventing literal lists where some members are literals and others are not. For example:

@external
def foo() -> uint256[2]:
    a: uint256 = 123
    return [a, 2]

This would fail because in parser/stmt.py the literal 2 is interpreted as an int128. However, we know this to be incorrect because we have already completed the type checking pass.

How I did it

Remove the now-redundant check that was blocking this at compile-time. I did some minor voodoo to retain the type information within parser so that nothing else breaks later down the line, and left a comment to prevent confusion later when we refactor types out altogether.

How to verify it

Check that tests are passing.

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