Remove deprecated max_connections config setting

[atoulme]

Component(s)

No response

Describe the issue you're reporting

This affects the following components:

• exporter/signalfxexporter
• exporter/splunkhecexporter

The max_connections config setting were deprecated with #16837 and #16838. It has been 10 months since these changes have been made and it is time to remove this configuration setting altogether.

[github-actions]

Pinging code owners for exporter/splunkhec: @atoulme @dmitryax. See Adding Labels via Comments if you do not have permissions to add labels yourself.