tint: Re-enable doxygen

Remove the default override for TINT_BUILD_DOCS.
Update the Doxyfile to only consider Tint.
Re-enable Kokoro checks.

Fixed: tint:1498
Bug: dawn:1339
Change-Id: I8bf5d7085452b42a6ed564c0f86ed7cbd85128b0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86381
Kokoro-Run: Ben Clayton <bclayton@google.com>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/Doxyfile b/Doxyfile
index 08eac60..c28c268 100644
--- a/Doxyfile
+++ b/Doxyfile
@@ -786,11 +786,8 @@
 # Note: If this tag is empty the current directory is searched.
 
 INPUT                  = CODE_OF_CONDUCT.md \
-                         src/tint/fuzzers/tint_spirv_tools_fuzzer \
-                         src \
-                         tools/src \
-                         src/tint/fuzzers/tint_spirv_tools_fuzzer \
-                         src/tint/fuzzers/tint_ast_fuzzer
+                         src/tint \
+                         tools/src
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses