fix ovn-bridge-mappings deletion

[zhangzujian]

• Make sure you have followed Kube-OVN Code Style.

• Bug fixes

Which issue(s) this PR fixes:

Fixes incorrect ovn-bridge-mappings after removing a non-existent entry.

[github-actions]

• The commit message should be more descriptive and informative.
• There are some unused variables in the code, they should be removed to improve readability.
• The function ovsCleanProviderNetwork has a lot of duplicated code. It can be refactored to reduce the number of lines and make it more readable.
• The function updateOvnMapping is not used anymore and can be removed.
• The function decodeOvnMappings can be simplified by using strings.SplitN instead of strings.IndexRune.
• The function encodeOvnMappings can be simplified by using strings.Builder.
• The function getOvnMappings can return an empty map instead of nil when there is an error.
• The function setOvnMappings can be simplified by using if len(mappings) == 0 instead of if s := encodeOvnMappings(mappings); len(s) == 0.
• The function addOvnMapping can be simplified by using if mappings[key] == value || !overwrite { return nil }.
• The function removeOvnMapping can be simplified by using if len(mappings) == length { return nil }.
• The function initProviderChassisMac can be simplified by using addOvnMapping instead of ovs.Exec.

[github-actions]

• The commit message should be more descriptive and informative.
• There are no potential bugs in the code patch diff.
• The format of the code is consistent with the rest of the project.
• There are some performance issues that can be improved. For example, the function getOvnMappings can be optimized to avoid unnecessary string operations.
• The code can be further improved by using constants for the external-ids names.
• The function addOvnMapping should have a better name since it can also update an existing mapping.
• The function removeOvnMapping should return an error if the key does not exist in the mappings.