config: Remove some "UNREACHABLE" UB footguns

Every branch of these long if/else if/else chains returns, so the
compiler can figure out on its own that the last line is never reached.

If, for some reason, one of the branches does not return (as was the
case after 205f1f7, fixed by b22322b), this would usually result in a compiler
error ("control reaches end of non-void function"), but adding UNREACHABLE
transforms this into silent undefined behaviour.

config.c

@@ -1012,8 +1012,6 @@ parse_section_main(struct context *ctx)
         LOG_CONTEXTUAL_ERR("not a valid option: %s", key);
         return false;
     }
-
-    UNREACHABLE();
 }
 
 static bool
@@ -2340,8 +2338,6 @@ parse_section_tweak(struct context *ctx)
         LOG_CONTEXTUAL_ERR("not a valid option: %s", key);
         return false;
     }
-
-    UNREACHABLE();
 }
 
 static bool