app: remove the "confirm-on-close" preference

the warning dialog cannot be disabled any longer now.

(cherry picked from commit e2c8a60665499cc945838fbd5a99964b09ec8892)
app: set GIMP_CONFIG_PARAM_IGNORE on "confirm-on-close"

(cherry picked from commit bfb875755976cd4b2cd4b02d5f3a32ebaf24642f)