SpeculationRules: Make tags non-optional in SpeculationRulesParams

The speculation rules tags is enabled by default from M136. After that,
tags should always be non-empty, as even when no tags are specified, it
should contain the null tag.

For code simplification, this CL removes implementation that was used
for handling the empty case. Also this CL removes the
SpeculationRulesTag feature flag.

Bug: 381687257
Change-Id: I944cd7ba66c660fd6671bfa787967e960f21a528
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6506041
Reviewed-by: Huanpo Lin <robertlin@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454910}