Remove unnecessary import #18
Description
ExternArgument is now exported in openqasm. We can remove https://github.com/openqasm/openpulse-python/blob/5b5c7ae7af352b0ae2cca16e311fbd4114d999cd/source/openpulse/openpulse/ast.py#LL21C2-L21C2
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