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_perlsfio.c

@@ -51,7 +51,7 @@ sfdcnewvim(void)
 {
     Sfdisc_t	*disc;
 
-    disc = (Sfdisc_t *)alloc((unsigned)sizeof(Sfdisc_t));
+    disc = (Sfdisc_t *)alloc(sizeof(Sfdisc_t));
     if (disc == NULL)
 	return NULL;