The timing restrictions code is not in use and not updated for ESSL3,
so it is better to remove it to make refactoring the AST easier.

It can also be argued that perfect prevention of shader timing attacks
is not feasible due to factors that are not under control of ANGLE,
such as fixed function color compression in GPUs. Such color
compression may make the use of texture bandwidth and thus performance
dependent on the content of a texture regardless of whether a
compressed format is chosen through the API.

SH_DEPENDENCY_GRAPH flag that could only be active together with the
timing restrictions flag is also removed, along with all the code that
was supporting it. The newer CallDAG code is used for different
purposes and is kept.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I2cd10e18df366e8e43f7c3af1ca12d2a4bfb2007
Reviewed-on: https://chromium-review.googlesource.com/384511
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>