[ntp footer] De-duplicate BUILD definitions of new_tab_footer_ui.cc/h.

A CL [1] added an entry for these to c/b/u/BUILD.gn, but they are
already defined by c/b/u/w/new_tab_footer/BUILD.gn[2].

[1]https://chromium-review.googlesource.com/c/chromium/src/+/6484395
[2]https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/new_tab_footer/BUILD.gn;drc=cde700d2fdbb34003e477e5ad2f07f2cd8f3b0f0;l=13

Change-Id: Iffd866890a10f77136b1cabb3858d49740e19493
Bug: 399177848
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6614897
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Reviewed-by: Jennifer Serrano <jennserrano@google.com>
Cr-Commit-Position: refs/heads/main@{#1468702}