[Home] Remove ChromeHomeModernLayout flag

Remove the flag controlling whether modern is enabled since modern is
always enabled when Chrome Home is enabled.

BUG=

Change-Id: I44d1a8d0556628756555e021d3bb5c314928a809
Reviewed-on: https://chromium-review.googlesource.com/671817
Commit-Queue: Theresa <[email protected]>
Reviewed-by: Bernhard Bauer <[email protected]>
Reviewed-by: Nicolas Dossou-Gbété <[email protected]>
Reviewed-by: Matthew Jones <[email protected]>
Cr-Commit-Position: refs/heads/master@{#503468}
57 files changed