Remove the "Incomplete adapter sequence" warning

[marcelm]

Cutadapt is used quite often to trim primer sequences, in which case it is expected that the "adapter" (primer) is not preceded/followed by a random nucleotide, but a specific one.

The warning was added to prevent people from providing an Illumina adapter that is missing the initial A. Instead, we could directly check for the Illumina adapter sequence (and only that one).

cf #779