How to clear conditional formatting (not all formatting) in Google Apps script

Google Apps Scripts now supports removing conditional formatting using clearConditionalFormatRules

var sheet = SpreadsheetApp.getActiveSheet();
sheet.clearConditionalFormatRules();

https://developers.google.com/apps-script/reference/spreadsheet/sheet#clearconditionalformatrules

This is possible with Google Sheets API v4, which Apps Script can access via Advanced Sheets Service (note that it must be enabled before use, as the linked page instructs). Here is a script that deletes all conditional formatting rules in Sheet1 of the current spreadsheet (you'll want to loop over sheets, etc).

function clearSheet() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var ssId = ss.getId();
  var sheet = ss.getSheetByName("Sheet1");
  var sheetId = sheet.getSheetId(); 
  var format_req = {
    "requests": [{
      "deleteConditionalFormatRule": { 
        "index": 0,
        "sheetId": sheetId
      }
    }]
  };
  var string_req = JSON.stringify(format_req);
  while (true) {
    try {
      Sheets.Spreadsheets.batchUpdate(string_req, ssId);
    }
    catch(e) {
      break;
    }
  }
}

Each conditional format rule has a 0-based "index". Deleting the rule with index 0 causes other indices to decrease by 1. So the loop continues deleting index = 0 rule until there isn't one, and an error is thrown (and caught, exiting the loop).

This is a weird way of sending requests: I'd much rather send one batch request like this:

  var format_req = {
    "requests": [
    {
      "deleteConditionalFormatRule": { 
        "index": 0,
        "sheetId": sheetId
      }
    },
    {
      "deleteConditionalFormatRule": { 
        "index": 1,
        "sheetId": sheetId
      }
    }]
  };

but to do this, one must know how many conditional formatting rules are there (and I don't see how one would find out). If you ask for more rules to be deleted than exist in the sheet, the entire request fails and nothing is deleted.

Without advanced service

With plain Apps Script, the best one can do is this:

  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getRange(1, 1, sheet.getMaxRows(), sheet.getMaxColumns());
  var backgrounds = range.getBackgrounds(); 
  var fontColors = range.getFontColor(); 
  var fontFamilies = range.getFontFamilies();
  // ... other get methods from https://developers.google.com/apps-script/reference/spreadsheet/range 

  // tricky part: modify the backgrounds, replacing the colors used in conditional formatting by white 

  range.clearFormat();
  range.setBackgrounds(backgrounds)
       .setFontColors(fontColors)
       .setFontFamilies(fontFamilies)
    // .set  other things

Here I am assuming that conditional formatting affects only cell backgrounds. If one is unable to filter the background colors (which requires knowing exactly what colors were used in conditional formatting rules), the effects of conditional formatting will become ordinary background color, which is very undesirable... it may be better to forego setting the background colors at all.