[TSVB] Y-axis has the format of the first series from a group

[DianaDerevyankina]

Kibana version: 7.9.1

Elasticsearch version: 7.9.1

Description of the problem including expected versus actual behavior:

When creating a TSVB visualization with some different series, each with their field formatting, Y-axis displays the format of the first series.
As Y-axis shows values for the whole group, not but only the first series, it might be a good idea to leave its values without any formatting in this case.

Steps to reproduce:

1. Create a TSVB visualization
2. Create a first series and customize its format
3. Create some other series and customize its format with something different

Screenshots:

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)