(Accessibility) Remove instances of deprecated EuiKeyboardAccessible

[myasonik]

EuiKeyboardAccessible was deprecated because it was abused more often than it was used successfully so we need to remove it from Kibana before December 2020.

It's used 3 times throughout monitoring each presenting different issues:

1. https://github.com/elastic/kibana/blob/master/x-pack/plugins/monitoring/public/components/chart/horizontal_legend.js#L70-L94
   Here it wraps a <button> which ends up with redundant structure with an extra tab stop:
   <div tabindex={0} role="button><button>{...}</button></div>. Here you should be able to remove <EuiKeyboardAccessible> without any further changes.

2. https://github.com/elastic/kibana/blob/master/x-pack/plugins/monitoring/public/components/elasticsearch/nodes/cells.js#L85-L96
   Here it wraps EuiIcon that has an onClick assigned to it. EuiButtonIcon will serve this usecase better.

3. https://github.com/elastic/kibana/blob/master/x-pack/plugins/monitoring/public/components/elasticsearch/shard_allocation/components/assigned.js#L56-L67
   Here it wraps a <a> to, it seems from the comment, to change the styles of it. Changing styles should be done with CSS, not with accessibility-focused components. EuiLink supports several colors to render links as.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[elastic-vault-github-plugin-prod]

@elastic/kibana-accessibility, this issue was not assigned to a specific WCAG category. Please update the issue description.

[alexwizp]

Obsolete, EuiKeyboardAccessible has been completely removed from Kibana. No more references