patch 8.1.1384: using "int" for alloc() often results in compiler warnings

Problem:    Using "int" for alloc() often results in compiler warnings.
Solution:   Use "size_t" and remove type casts.  Remove alloc_check(), Vim
            only works with 32 bit ints anyway.

src/if_python3.c

@@ -1629,7 +1629,7 @@ LineToString(const char *str)
     Py_ssize_t len = strlen(str);
     char *tmp,*p;
 
-    tmp = (char *)alloc((unsigned)(len+1));
+    tmp = (char *)alloc(len + 1);
     p = tmp;
     if (p == NULL)
     {