patch 8.2.4241: some type casts are redundant

Problem:    Some type casts are redundant.
Solution:   Remove the type casts. (closes #9643)
diff --git a/src/spellfile.c b/src/spellfile.c
index af6c6ef..b5c6693 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -3618,7 +3618,7 @@
 	}
 
 	// Store the word in the hashtable to be able to find duplicates.
-	dw = (char_u *)getroom_save(spin, w);
+	dw = getroom_save(spin, w);
 	if (dw == NULL)
 	{
 	    retval = FAIL;