cpan/various.xs: don't force "C" linkage on system headers

This produced errors with MSVC C++ builds like:

C:\Program Files (x86)\Windows Kits\10\include\10.0.22000.0\um\wspiapi.h(53):
error C2894: templates cannot be declared to have 'C' linkage

cpan/Digest-MD5/MD5.xs

@@ -32,16 +32,10 @@
  * documentation and/or software.
  */
 
-#ifdef __cplusplus
-extern "C" {
-#endif
 #define PERL_NO_GET_CONTEXT     /* we want efficiency */
 #include "EXTERN.h"
 #include "perl.h"
 #include "XSUB.h"
-#ifdef __cplusplus
-}
-#endif
 
 #ifndef PERL_UNUSED_VAR
 # define PERL_UNUSED_VAR(x) ((void)x)

cpan/Time-Piece/Piece.xs

@@ -1,15 +1,8 @@
-#ifdef __cplusplus
-extern "C" {
-#endif
 #define PERL_NO_GET_CONTEXT
 #include "EXTERN.h"
 #include "perl.h"
 #include "XSUB.h"
 #include <time.h>
-#ifdef __cplusplus
-}
-#endif
-
 
 #define    DAYS_PER_YEAR    365
 #define    DAYS_PER_QYEAR    (4*DAYS_PER_YEAR+1)

dist/Time-HiRes/HiRes.xs

@@ -11,9 +11,6 @@
  * it under the same terms as Perl itself.
  */
 
-#ifdef __cplusplus
-extern "C" {
-#endif
 #define PERL_NO_GET_CONTEXT
 #include "EXTERN.h"
 #include "perl.h"
@@ -40,9 +37,6 @@ extern "C" {
 #if defined(TIME_HIRES_CLOCK_GETTIME_SYSCALL) || defined(TIME_HIRES_CLOCK_GETRES_SYSCALL)
 #  include <syscall.h>
 #endif
-#ifdef __cplusplus
-}
-#endif
 
 #ifndef GCC_DIAG_IGNORE
 #  define GCC_DIAG_IGNORE(x)