Clean up obsolete code in BlockBasedTable::PrefetchIndexAndFilterBlocks #3896

Job	Run time
build-linux-cmake-with-folly	28m 32s
check-format-and-targets	22s
build-linux	16m 57s
build-linux-cmake-mingw	12m 20s
build-linux-cmake-with-benchmark	19m 31s
build-linux-cmake-with-folly-coroutines	32m 21s
build-linux-cmake-with-folly-lite-no-test	7m 55s
build-linux-encrypted_env-no_compression	16m 31s
build-linux-make-with-folly	29m 3s
build-linux-make-with-folly-lite-no-test	9m 23s
build-examples	10m 58s
build-fuzzers	8m 38s
build-linux-clang10-clang-analyze	17m 31s
build-linux-clang-13-no_test_run	7m 20s
build-linux-clang-no_test_run	12m 10s
build-linux-gcc-10-cxx20-no_test_run	9m 39s
build-linux-gcc-11-no_test_run	10m 14s
build-linux-gcc-8-no_test_run	8m 51s
build-linux-release	14m 9s
build-linux-release-rtti	12m 47s
build-linux-arm	15m 0s
build-linux-clang10-asan	17m 14s
build-linux-clang13-mini-tsan	24m 53s
build-linux-clang10-ubsan	17m 3s
build-linux-java	15m 29s
build-linux-java-pmd	12m 35s
build-linux-java-static	12m 4s
build-linux-mini-crashtest	29m 49s
build-linux-unity-and-headers	13m 38s
build-macos	26m 47s
build-linux-static_lib-alt_namespace-status_checked	19m 46s
build-macos-cmake (true)	48m 2s
build-macos-cmake (false)	43m 36s
build-macos-java	18m 31s
build-macos-java-static	25m 21s
build-macos-java-static-universal	23m 34s
build-windows-vs2019	52m 46s
	11h 41m 20s

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