[ruby/json] Drop the leftovers of runtime SSE2 detection

Neither the `__builtin_cpu_supports` probe in conf.rb nor `cpuid.h` has a use now that SSE2 is assumed on x86-64. The probe kept SIMD off on GCC older than 4.8, and the `HAVE_CPUID_H` guard broke builds that have `x86intrin.h` but not `cpuid.h` with a redefinition of `find_simd_implementation`.

https://github.com/ruby/json/commit/fae76b9eb4

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ext/json/simd/conf.rb

@@ -1,16 +1,15 @@
 case RbConfig::CONFIG['host_cpu']
 when /^(arm|aarch64)/
   # Try to compile a small program using NEON instructions
-  header, type, init, extra = 'arm_neon.h', 'uint8x16_t', 'vdupq_n_u8(32)', nil
+  header, type, init = 'arm_neon.h', 'uint8x16_t', 'vdupq_n_u8(32)'
 when /^(x86_64|x64)/
-  header, type, init, extra = 'x86intrin.h', '__m128i', '_mm_set1_epi8(32)', 'if (__builtin_cpu_supports("sse2")) { printf("OK"); }'
+  header, type, init = 'x86intrin.h', '__m128i', '_mm_set1_epi8(32)'
 end
 if header
   if have_header(header) && try_compile(<<~SRC, '-Werror=implicit-function-declaration')
       #{cpp_include(header)}
       int main(int argc, char **argv) {
         #{type} test = #{init};
-        #{extra}
         if (argc > 100000) printf("%p", &test);
         return 0;
       }
@@ -20,5 +19,3 @@ if header
     puts "Disable SIMD"
   end
 end
-
-have_header('cpuid.h')

ext/json/simd/simd.h

@@ -143,7 +143,6 @@ ALWAYS_INLINE(static) int string_scan_simd_neon(const char **ptr, const char *en
 #define HAVE_SIMD 1
 #define HAVE_SIMD_SSE2 1
 
-#ifdef HAVE_CPUID_H
 #define FIND_SIMD_IMPLEMENTATION_DEFINED 1
 
 #if defined(__clang__) || defined(__GNUC__)
@@ -182,9 +181,6 @@ ALWAYS_INLINE(static) TARGET_SSE2 int string_scan_simd_sse2(const char **ptr, co
     return 0;
 }
 
-#include <cpuid.h>
-#endif /* HAVE_CPUID_H */
-
 static inline SIMD_Implementation find_simd_implementation(void)
 {
     // SSE2 is part of the x86-64 baseline.