Remove cplusplus guards in internal headers

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5105)

crypto/ec/curve448/curve448utils.h

@@ -15,10 +15,6 @@
 
 # include <openssl/e_os2.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /*
  * Internal word types. Somewhat tricky.  This could be decided separately per
  * platform.  However, the structs do need to be all the same size and
@@ -79,8 +75,4 @@ static ossl_inline c448_error_t c448_succeed_if(c448_bool_t x)
     return (c448_error_t) x;
 }
 
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
 #endif                          /* __C448_COMMON_H__ */

crypto/ec/curve448/ed448.h

@@ -15,10 +15,6 @@
 
 # include "point_448.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /* Number of bytes in an EdDSA public key. */
 # define EDDSA_448_PUBLIC_BYTES 57
 
@@ -200,8 +196,4 @@ c448_error_t c448_ed448_convert_private_key_to_x448(
                             uint8_t x[X448_PRIVATE_BYTES],
                             const uint8_t ed[EDDSA_448_PRIVATE_BYTES]);
 
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
 #endif                          /* __C448_ED448_H__ */

crypto/ec/curve448/point_448.h

@@ -16,9 +16,6 @@
 # include "curve448utils.h"
 # include "field.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
 
 # define C448_SCALAR_LIMBS ((446-1)/C448_WORD_BITS+1)
 
@@ -283,8 +280,4 @@ void curve448_scalar_destroy(curve448_scalar_t scalar);
 /* Overwrite point with zeros. */
 void curve448_point_destroy(curve448_point_t point);
 
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
 #endif                          /* __C448_POINT_448_H__ */