How to get that "silent u" for certain lyrics?

There's no setting to make it silent. The only way to make it 'silent' is to make the 'su' in 'desu' a really short note, so that you can only hear the 's'. This is a common technique used in Japanese singing synthesis programs, including UTAU, to make notes like 'tsu', 'su' and sometimes 'fu' silent.
This is an example:

Click on the tilde button in the bottom left corner to display note envelopes. You can edit a note's envelope by either right clicking and selecting Envelope, or with CTRL+Y.

The gray vertical line shows where the consonant ends and the vowel begins. You can click and drag the last red square in the bottom right and move it to the left, so that the volume cuts off at the end of the consonant.