Subversion: Force deletion of files

vcsDelete(...) is used after a file is already deleted from the
filesystem (and after asking the user), so it is safe to --force
the removal in svn.

Task-number: QTCREATORBUG-13271
Change-Id: Id3c076987f6518efb186a04d31855b189cfd52ab
Reviewed-by: Orgad Shaneh <orgads@gmail.com>