[cleanup] Remove FallbackTheme and friends.

LayoutTheme::ShouldUseFallbackTheme should always be returning false,
as LayoutThemeMobile was the only place returning true - presumably for
an iOS configuration at some point.

Removes all FallbackTheme related methods, and classes.

There should be no behaviour change.

Change-Id: I1d336016a1823a53a977ad764706d68fd19ff9bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363846
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800391}