Remove sfiera@ from OWNERS files

Not working on chromium anymore, so probably not interested in doing
any more code reviews.

Bug: none
Change-Id: I5dd7adbba724fb9e33803a2a3478d32c9926506f
Reviewed-on: https://chromium-review.googlesource.com/c/1355128Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612384}

components/ntp_tiles/OWNERS

 fhorschig@chromium.org
 jkrcal@chromium.org
 mastiz@chromium.org
-sfiera@chromium.org
 treib@chromium.org

components/search/OWNERS

 kristipark@chromium.org
 mathp@chromium.org
 ramyan@chromium.org
-sfiera@chromium.org
 treib@chromium.org
 
 # Original implementors of most of the code, but not active in Chromium anymore:

ios/chrome/browser/ntp_tiles/OWNERS

-sfiera@chromium.org
 file://components/ntp_tiles/OWNERS
 
 # TEAM: ios-directory-owners@chromium.org