[Sheriff] Remove platform-specific expectation file for webxr IDL

It seems like this is causing consistent failures on Mac10.10 tests; not
sure why it was created by the WPT importer in the first place.

https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests/48035

TBR=klausw@chromium.org

Bug: None
Change-Id: I89cf2d8c0e7ec47f80952310b746c3a904bef863
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876889Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708737}

third_party/blink/web_tests/platform/mac-mac10.10/external/wpt/webxr/ar-module/idlharness.https.window-expected.txt

-This is a testharness.js-based test.
-PASS idl_test setup
-FAIL idl_test validation Validation error at line 19 in webxr, inside `enum XRSessionMode`:
-enum XRSessionMode {
-     ^ The name "XRSessionMode" of type "enum" was already seen
-PASS Partial interface XRSession: original interface defined
-PASS Partial interface XRSession: member names are unique
-PASS XRSession interface: attribute environmentBlendMode
-Harness: the test ran to completion.
-