Remove unused return value from _check() method

[0xjei]

Description

This PR removes the unused return value from _check() method from Excubia abstract contract. See this comment for more.

Related Issue(s)

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Other information

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run yarn format and yarn compile without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cedoor]

LGTM! Just one comment (not related to your code).

Looks like there's been an error while compiling contracts.