CI: Remove goveralls -repotoken argument.

[cpu]

Leaving it in causes external PRs to fail because the encrypted COVERALLS_TOKEN env var isn't available and an empty -repotoken argument is invalid (See e.g. https://travis-ci.org/letsencrypt/ct-woodpecker/builds/469815134).

We don't need the -repotoken at all: it is only useful for private Github repositories.

[coveralls]

Pull Request Test Coverage Report for Build 507

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.809%

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Totals
Change from base Build 499:	0.0%
Covered Lines:	811
Relevant Lines:	1196

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💛 - Coveralls