Auto merge of #15064 - aneeshusa:remove-webrender-opt, r=Ms2ger

Remove -w flag now that WebRender is always used

Also remove the obsolete `--cpu` and `--gpu` renderer flags,
which also are no longer used.

Update tests and wptrunner to not pass these flags.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13761 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because the tests are updated to no longer pass these flags

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components/config/opts.rs

@@ -573,8 +573,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
     let (app_name, args) = args.split_first().unwrap();
 
     let mut opts = Options::new();
-    opts.optflag("c", "cpu", "CPU painting");
-    opts.optflag("g", "gpu", "GPU painting");
     opts.optopt("o", "output", "Output file", "output.png");
     opts.optopt("s", "size", "Size of tiles", "512");
     opts.optopt("", "device-pixel-ratio", "Device pixels per px", "");
@@ -620,7 +618,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
     opts.optmulti("", "pref",
                   "A preference to set to enable", "dom.mozbrowser.enabled");
     opts.optflag("b", "no-native-titlebar", "Do not use native titlebar");
-    opts.optflag("w", "webrender", "Use webrender backend");
     opts.optopt("G", "graphics", "Select graphics backend (gl or es2)", "gl");
     opts.optopt("", "config-dir",
                     "config directory following xdg spec on linux platform", "");