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/hashtab.c

@@ -400,8 +400,7 @@ hash_may_resize(
     else
     {
 	/* Allocate an array. */
-	newarray = (hashitem_T *)alloc((unsigned)
-					      (sizeof(hashitem_T) * newsize));
+	newarray = (hashitem_T *)alloc(sizeof(hashitem_T) * newsize);
 	if (newarray == NULL)
 	{
 	    /* Out of memory.  When there are NULL items still return OK.