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/ex_cmds2.c b/src/ex_cmds2.c
index 5945203..c56e305 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -2366,7 +2366,7 @@
     }
     else
     {
-	buf = alloc((unsigned)(STRLEN(eap->arg) + 14));
+	buf = alloc(STRLEN(eap->arg) + 14);
 	if (buf != NULL)
 	{
 	    if (eap->forceit)