refactor(mission_planner): remove redundant is_reroute check

[veqcc]

Description

This is a fix based on CppCheck warning

planning/mission_planner/src/mission_planner/mission_planner.cpp:253:18: style: Condition 'is_reroute' is always true [knownConditionTrueFalse]
    change_state(is_reroute ? RouteState::SET : RouteState::UNSET);
                 ^
planning/mission_planner/src/mission_planner/mission_planner.cpp:251:7: note: Assuming that condition 'is_reroute' is not redundant
  if (is_reroute && !check_reroute_safety(*current_route_, route)) {
      ^
planning/mission_planner/src/mission_planner/mission_planner.cpp:253:18: note: Condition 'is_reroute' is always true
    change_state(is_reroute ? RouteState::SET : RouteState::UNSET);
                 ^
planning/mission_planner/src/mission_planner/mission_planner.cpp:301:18: style: Condition 'is_reroute' is always true [knownConditionTrueFalse]
    change_state(is_reroute ? RouteState::SET : RouteState::UNSET);
                 ^
planning/mission_planner/src/mission_planner/mission_planner.cpp:299:7: note: Assuming that condition 'is_reroute' is not redundant
  if (is_reroute && !check_reroute_safety(*current_route_, route)) {
      ^
planning/mission_planner/src/mission_planner/mission_planner.cpp:301:18: note: Condition 'is_reroute' is always true
    change_state(is_reroute ? RouteState::SET : RouteState::UNSET);
                 ^

Tests performed

No

Effects on system behavior

Interface changes

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.
• The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• The PR follows the pull request guidelines.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.