Remove enable_ppapi from build files in content/child

enable_ppapi is always false with nacl being gone.
Remove some resulting dead code from content/child

This CL was uploaded by git cl split.

R=dom@chromium.org

Bug: 40511450
Change-Id: Ife443015d1a70681dda3fe93bd27e4b4e7ab55e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6668294
Reviewed-by: Dominic Farolino <dom@chromium.org>
Commit-Queue: Fabian Sommer <fabiansommer@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Auto-Submit: Fabian Sommer <fabiansommer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1477980}