fix(gatsby-plugin-sitemap): Removed redundant check for siteUrl

[dhakerShivnarayan]

Description

Related Issues

[wardpeet]

less code is always better. Thank you! 💯

[gatsbot]

Holy buckets, @dhakerShivnarayan — we just merged your PR to Gatsby! 💪💜

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[wardpeet]

Published in gatsby-plugin-sitemap@2.2.13