`insertSelectPlaceholder` is an unexported const set at true. No code changes this nor can any build environment change it.
Therefore we should remove it and the associated dead code.
Close#2146
Signed-off-by: Andrew Thornton <art27@cantab.net>
Reviewed-on: https://gitea.com/xorm/xorm/pulls/2151
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-committed-by: Andrew Thornton <art27@cantab.net>