Remove the default for replica-net-timeout

[GuptaManan100]

Description

As described in #15937 (comment), this PR needs to revert the changes in #15663 and #15689. We'll reintroduce the default change in next release.

Related Issue(s)

• Fixes Bug Report: Changing default of replica_net_timeout can cause issues while upgrading #15937

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[vitess-bot]

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[mattlord]

So this ended up being a problem? #15663 (review) Or it was something else?

[GuptaManan100]

@mattlord Yes exactly! It was exactly that, if we restart a running mysql instance with the new my.cnf then the replica_net_timeout value gets changed, but the hearbeat_interval does not.