mesa: remove dead code in _mesa_error

Remove early and unused snprintf and where[] string.

src/mesa/main/imports.c

    }
 
    if (debug) {
-      va_list args;
-      char where[MAXSTRING];
       const char *errstr;
 
-      va_start( args, fmtString );  
-      vsnprintf( where, MAXSTRING, fmtString, args );
-      va_end( args );
-
       switch (error) {
 	 case GL_NO_ERROR:
 	    errstr = "GL_NO_ERROR";