Fix comment about AVX-512 popcount support.

Since commit f78667bd91, we've used __attribute__((target(...)))
instead of extra compiler flags for AVX-512 support, but this
comment still says that we put the code in a separate file because
it might require extra compiler flags.  Let's just remove that part
of the comment.

src/include/port/pg_bitutils.h

@@ -307,9 +307,7 @@ extern PGDLLIMPORT uint64 (*pg_popcount_masked_optimized) (const char *buf, int
 
 /*
  * We can also try to use the AVX-512 popcount instruction on some systems.
- * The implementation of that is located in its own file because it may
- * require special compiler flags that we don't want to apply to any other
- * files.
+ * The implementation of that is located in its own file.
  */
 #ifdef USE_AVX512_POPCNT_WITH_RUNTIME_CHECK
 extern bool pg_popcount_avx512_available(void);