Dynamic data validation in Apps Script

onEdit.gs

function onEdit(e) {
  var row = e.range.getRow();
  var col = e.range.getColumn();
  
  // If the edited column was 2 or 3, do the next part. Ignore other changes
  if(col == 2 || col == 3) {
    var value = sheet.getRange(row, col).getValue();
    updateList(row,col, value);
  }
}

setValidation.gs

var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheetByName("ChoiceSheet");
var dates = ss.getSheetByName("Dates");

function setValidation() {
  var cells = sheet.getRange("B2:C15");
  var rules = dates.getRange("A:A");
  var validation = SpreadsheetApp.newDataValidation().requireValueInRange(rules).build();
  cells.setDataValidation(validation);
}

updateValidation.gs

function updateValidation(row,col,value) {
  var data = dates.getDataRange().getValues();
  
  // Looping backwards lets you cleanly delete rows
  for(var i=data.length-1; i>=0; i--) {
    if(data[i].toString() == value) {
      dates.deleteRow((i+1));
    }
  }
  // Remove the validation from the edited cell so it doesn't show an error
  sheet.getRange(row, col).setDataValidation(null);
}