fixed gpu implementation of [lbboundary delete]

## a simple testcase
require_feature "LB_BOUNDARIES_GPU"

setmd box_l  10 10 2 ; setmd skin 0.4
thermostat off ; setmd time_step 1.0

lbfluid gpu  agrid 1 dens 1.0 visc 1.0 tau 1.0
lbboundary wall normal 0  1 0 dist 1  velocity 1. 0. 0.

integrate 2000
# this should output  1.0  
puts "[lindex [lbnode 0 1 0 print u] 0 ] "

lbboundary delete

lbboundary wall normal 0  1 0 dist 1  velocity -1. 0. 0.
integrate 2000
# this should output  -1.0   but in the old code 
# the boundary is not reset, and keeps printing  1.0 
puts "[lindex [lbnode 0 1 0 print u] 0 ] "