[VecOps] Remove outdated IsSmall helper function in tests

This is a follow-up on 2605710, which is required to make the
`vecops_rvec.cxx` tests pass on ARM64 with GCC 14.

That commit changed from using the `ROOT::Detail::VecOps::` namespace
explicitly for `IsSmall` to a `using` statement. However, there was also
a remnant `IsSmall` function with an outdated implementation in the test
file itself, which was accidentally picked up by the tests then.

Author: guitargeek

math/vecops/test/vecops_rvec.cxx

@@ -1493,13 +1493,6 @@ TEST(VecOps, Construct)
    EXPECT_TRUE(fourVects[2] == ref2);
 }
 
-bool IsSmall(const RVec<int> &v)
-{
-   // the first array element is right after the 3 data members of SmallVectorBase
-   return reinterpret_cast<std::uintptr_t>(v.begin()) - reinterpret_cast<std::uintptr_t>(&v) ==
-          sizeof(void *) + 2 * sizeof(int);
-}
-
 // this is a regression test for https://github.com/root-project/root/issues/6796
 TEST(VecOps, MemoryAdoptionAndClear)
 {