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.
diff --git a/src/channel.c b/src/channel.c
index 31c9bbe..1564128 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -2024,7 +2024,7 @@
 	}
 	else
 	{
-	    item = (jsonq_T *)alloc((unsigned)sizeof(jsonq_T));
+	    item = (jsonq_T *)alloc(sizeof(jsonq_T));
 	    if (item == NULL)
 		clear_tv(&listtv);
 	    else
@@ -2223,7 +2223,7 @@
 	/* append after the last item that was pushed back */
 	item = item->jq_next;
 
-    newitem = (jsonq_T *)alloc((unsigned)sizeof(jsonq_T));
+    newitem = (jsonq_T *)alloc(sizeof(jsonq_T));
     if (newitem == NULL)
 	clear_tv(rettv);
     else