[EuiTabs] Remove 'xl' size

[ryankeairns]

Two reasons for this request:

1. Subjectively, it is an atypical font size for tabs (see below)[a].
2. Objectively, we don't use it in Kibana [b]

[a] The guidelines say to only use for page titles only, but I don't believe this is a justifiable recommendation.
[b] Search for usage with this regex <EuiTabs[ \w=\{\}>\(\)\n;".-:]* size="xl"

Note: I suppose this would be a breaking change for anyone out there is using it (seem unlikely; low impact), and we could suggest using l as the recommended fix.

[ryankeairns]

Remove the xl size and suggest to use l.
This seems non-controversial given its lack of known usage.