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GH-38339: [C++][CMake] Use transitive dependency for system GoogleTest (#38340)

### Rationale for this change

Our bundled GoogleTest has transitive dependency. So we can omit explicit `GTest::gtest` linking because `GTest::gtest_main`/`GTest::gmock` has transitive dependency.

But GoogleTest related targets found by CMake's `FindGTest.cmake` don't have transitive dependency. So our code base needs to link `GTest::gtest` explicitly.

We can remove needless links by setting transitive dependency to GoogleTest related targets found by CMake's `FindGTest.cmake`.

### What changes are included in this PR?

* Set transitive dependencies to `GTest::gtest_main` and `GTest::gmock`
* Remove needless links

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #38339

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>