Hide "Skip pairing" in case of single-end input reads in BWA-MEM

The "Skip pairing" option of BWA-MEM makes sense only when the input reads are paired-end. Hide the option when it is not so (parameter "Library" in UGENE 1.31).

Apply the modification to the "Map NGS Reads to Reference" dialog and to the BWA-MEM workflow element.

Also, improve the description of the parameter.