Remove OWNERs files from empty push_messaging directories

These should have been removed with CL1631403 and CL1641882 but
I forgot to do it, so let's remove them now to keep the tree clean.

Bug: 939943
Change-Id: Ie185baf8cf653e68d5f9a10bbc4a5b39982e631f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1655469
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668399}

content/renderer/push_messaging/OWNERS

-file://content/browser/push_messaging/OWNERS
-
-# TEAM: platform-capabilities@chromium.org
-# COMPONENT: Blink>PushAPI

third_party/blink/common/push_messaging/OWNERS

-file://third_party/blink/renderer/modules/push_messaging/OWNERS
-
-per-file *_mojom_traits*.*=set noparent
-per-file *_mojom_traits*.*=file://ipc/SECURITY_OWNERS
-
-# TEAM: platform-capabilities@chromium.org
-# COMPONENT: Blink>PushAPI