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/bootspec.c

@@ -960,7 +960,7 @@ int boot_config_augment_from_loader(
                 if (!c)
                         return log_oom();
 
-                STRV_FOREACH_PAIR(a, b, (char**) title_table)
+                STRV_FOREACH_PAIR(a, b, title_table)
                         if (streq(*a, *i)) {
                                 t = strdup(*b);
                                 if (!t)