WebUI: Remove some exceptions for definitions in validate_tsconfig

(1) Remove third_party/lit exception as it appears to be unnecessary
(2) Remove tools/typescript/definitions exception, so that UIs are
    required to list all definitions from this folder that they are
    using. For all UIs other than the ones using the dependency mapping,
    this fixes a potential build issue as these targets were depending
    on files but not listing them as inputs for gn.
(3) Only a couple of UIs were actually using the dependency mapping for
    tools/typescript/definitions introduced in
    https://chromium-review.googlesource.com/c/chromium/src/+/5315610
    so revert most of this change, in favor of the exception removal
    mentioned in (2). The approach in (2) is preferred because most
    untracked dependencies in tools/typescript/definitions came from
    .d.ts files importing other .d.ts files, rather than from imports
    accessed via path_mappings.

Bug: 326005022
Change-Id: Id102544c21b04b6efd55c3c19fabc992f3778efe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5333878


Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267378}