Revert "Update Reading mode field trial test config with latest experiments."

This reverts commit a83418377b7253b9c4d58507afe2dd9d487d4ef9.

Reason for revert:
LUCI Bisection has identified this change as the cause of a test failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/test-analysis/b/6325627889123328

Sample build with failed test: https://ci.chromium.org/b/8708562722914577473
Affected test(s):
[ninja://ios/chrome/test/earl_grey2:ios_chrome_ui_eg2tests_module/PostRestoreDefaultBrowserPromoTestCase/testPromoAppears](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fios%2Fchrome%2Ftest%2Fearl_grey2:ios_chrome_ui_eg2tests_module%2FPostRestoreDefaultBrowserPromoTestCase%2FtestPromoAppears?q=VHash%3A41b246b749ee4400)
[ninja://ios/chrome/test/earl_grey2:ios_chrome_ui_eg2tests_module/PostRestoreDefaultBrowserPromoTestCase/testPromoAppears](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fios%2Fchrome%2Ftest%2Fearl_grey2:ios_chrome_ui_eg2tests_module%2FPostRestoreDefaultBrowserPromoTestCase%2FtestPromoAppears?q=VHash%3A50b56437dde8882e)
[ninja://ios/chrome/test/earl_grey2:ios_chrome_ui_eg2tests_module/PostRestoreDefaultBrowserPromoTestCase/testPromoAppears](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fios%2Fchrome%2Ftest%2Fearl_grey2:ios_chrome_ui_eg2tests_module%2FPostRestoreDefaultBrowserPromoTestCase%2FtestPromoAppears?q=VHash%3Ad46fb01f0b498a72)

If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Ftest-analysis%2Fb%2F6325627889123328&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F6781040&type=BUG

Original change's description:
> Update Reading mode field trial test config with latest experiments.
>
> Removes IOSReaderModeMetrics experiment which is no longer active and
> adds the ReaderModeDefaultBrowserPromo feature.
>
> Bug: 426175025
> Change-Id: Icba1527edf3eb81f6525360cd6ab2072f95332b8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781040
> Reviewed-by: Quentin Pubert <qpubert@google.com>
> Auto-Submit: Nohemi Fernandez <fernandex@google.com>
> Commit-Queue: Nohemi Fernandez <fernandex@google.com>
> Cr-Commit-Position: refs/heads/main@{#1490691}
>

Bug: 426175025
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I8b69b40c715738a64d3c2a698adc52f51f00d9c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6779184
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Matthias Körber <koerber@google.com>
Owners-Override: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490818}