MDEV-8941 Compile on Solaris (SPARC) fails with errors in filamvct.cpp

There are a large number of uses of `strerror` in the codebase,
the local declaration in `storage/connect/tabvct.cpp` is the only one.

Given that none is needed elsewhere, I conclude that this instance can
simply be removed.

storage/connect/tabvct.cpp

@@ -71,11 +71,6 @@
 #include "tabvct.h"
 #include "valblk.h"
 
-#if defined(UNIX)
-//add dummy strerror   (NGC)
-char *strerror(int num);
-#endif   // UNIX
-
 /***********************************************************************/
 /*  External function.                                                 */
 /***********************************************************************/