Remove unnecessary status messages when detecting MPI continuations support

[msimberg]

We already print:

-- Performing Test PIKA_WITH_MPIX_CONTINUATIONS - Failed

when performing the feature test itself. I propose to remove the trailing:

-- MPIx Continuations not detected

since:

• it says the same as the previous line (admittedly a bit nicer, but still)
• we only print the Performing Test PIKA_WITH... line for all other feature checks

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ +0.02% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d766280)	18346	13763	75.02%
Head commit (73f090e)	18346 (+0)	13767 (+4)	75.04% (+0.02%)

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1250)	0	0	∅ (not applicable)

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