Fixed #2221: double attribution in print preview for leaflet

[mbarto]

Description

See title

Issues

• See title
  Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
double attribution in print preview for leaflet

What is the new behavior?
single attribution in print preview for leaflet

Does this PR introduce a breaking change? (check one with "x", remove the other)

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Coverage remained the same at 80.742% when pulling 2547437 on mbarto:print_double_attribution into d8d5b60 on geosolutions-it:master.