[ADT] Drop append_range test that doesn't compile with EXPENSIVE_CHECKS

The issue seems to be caused by the definition of `SafeIntIterator` and
is otherwise unrelated to the implementation of `append_range`.

We have a simialr test below, so this does not meaningfully decrease the
test coverage of `append_range`.

See the discussion in: https://reviews.llvm.org/D144420#4164373

Issue: #61122

Author: kuhar

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/StringRef.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -405,9 +404,6 @@ TEST(STLExtrasTest, AppendRange) {
   append_range(V, AppendVals2);
   EXPECT_THAT(V, ElementsAre(1, 2, 3, 4, 5));
 
-  append_range(V, llvm::seq(6, 8));
-  EXPECT_THAT(V, ElementsAre(1, 2, 3, 4, 5, 6, 7));
-
   std::string Str;
   append_range(Str, "abc");
   EXPECT_THAT(Str, ElementsAre('a', 'b', 'c', '\0'));