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/gui_motif.c b/src/gui_motif.c
index 4217339..1693df9 100644
--- a/src/gui_motif.c
+++ b/src/gui_motif.c
@@ -2538,7 +2538,7 @@
     for (p = buts; *p; ++p)
 	if (*p == DLG_BUTTON_SEP)
 	    ++butcount;
-    buttons = (Widget *)alloc((unsigned)(butcount * sizeof(Widget)));
+    buttons = (Widget *)alloc(butcount * sizeof(Widget));
     if (buttons == NULL)
     {
 	vim_free(buts);