GitLab

Remove declaration of FLAGS_use_http2_frame_decoder_adapter from spdy_flags.h
which is never used in the codebase, see [1].  Instead,
FLAGS_chromium_http2_flag_spdy_use_http2_frame_decoder_adapter is used
everywhere.

[1] https://cs.chromium.org/search/?q=FLAGS_use_http2_frame_decoder_adapter&ssfr=1&type=cs

Review-Url: https://codereview.chromium.org/2758523002
Cr-Commit-Position: refs/heads/master@{#457529}