Remove BlinkGenPropertyTrees feature flag and residue

Since the Blink runtime enabled feature (BlinkGenPropertyTrees) was
removed in d6c4a0cff4b083e7143eb4026351ea8c7450450a, this no longer does
anything (and would trigger a warning from
RuntimeEnabledFeatures::SetFeatureEnabledFromString).

Bug: 989251
Change-Id: I7658c7d99c2b8bd2f43c26ce86e288b61a0546ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753933
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#686819}
7 files changed