tree-wide: drop de-constifying casts for strv iteration

When the the iterator variable is declared automatically, it "inherits" the
const/non-const status from the argument. We don't need to cast a const
table to non-const. If we had a programming error and tried to modify the
string, the compiler could now catch this.

src/shared/conf-parser.c

@@ -497,7 +497,7 @@ static int config_parse_many_files(
         }
 
         /* First read the first found main config file. */
-        STRV_FOREACH(fn, (char**) conf_files) {
+        STRV_FOREACH(fn, conf_files) {
                 r = config_parse(NULL, *fn, NULL, sections, lookup, table, flags, userdata, &st);
                 if (r < 0)
                         return r;