Is it possible to withdraw a deletion vote?
Most probably not, but I want to be sure. (A few words about the motivation of not permitting this is also welcome.)
The votes to delete, undelete, and reopen cannot be retracted. There is a feature request to change this: Ability to retract reopen/delete votes
Possible explanation: these votes are cast far less often than close votes, and so the number of situations where a voter wishes to retract is proportionally smaller. In SE team's perception, it's not large enough to justify putting developer resources into this.
Things might change in the future... consider that the request to retract close votes was posted in June 2009 and implemented in July 2013.