Drop Trusty from WebKit Linux builders

I applied following command to renmae
$ git grep --name-only 'WebKit Linux Trusty ' testing/ infra/ | \
  xargs sed -i 's/WebKit Linux Trusty /WebKit Linux /g'

This is step 3 written in
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1341927

Bug: 954450
Change-Id: Ia29366bcdb60d7b28b44964a89d0033855866b8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635069
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664591}