cmake: Remove ENABLE_AVX2 from bitcoin-build-config.h

`ENABLE_AVX2` is already conditionally defined for the `bitcoin_crypto`
target, so defining it in `bitcoin-build-config.h` is redundant.

test/lint/test_runner/src/main.rs

@@ -619,7 +619,6 @@ fn lint_includes_build_config() -> LintResult {
                     // These are exceptions which don't use bitcoin-build-config.h, rather CMakeLists.txt adds
                     // these cppflags manually.
                     ":(exclude)src/crypto/sha256_arm_shani.cpp",
-                    ":(exclude)src/crypto/sha256_avx2.cpp",
                     ":(exclude)src/crypto/sha256_x86_shani.cpp",
                 ]),
         )