Remove the precompiled.cpp file which was producing a build warning,
and rename the header to make it clear we no longer support PCH.

BUG=angle:

Change-Id: I944081ae477b1478ca9c18924fb02bb920d420a7
Reviewed-on: https://chromium-review.googlesource.com/214187Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>