Fix reduction for full image in SSIM

[SkafteNicki]

What does this PR do?

Fixes #1186
Removes the reduction that is done in ssim when return_full_image=True, as the argument name indicates in this case no reduction should be applied.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1204 (ee59952) into master (bad3607) will decrease coverage by 52%.
The diff coverage is 67%.

@@           Coverage Diff            @@
##           master   #1204     +/-   ##
========================================
- Coverage      94%     42%    -52%     
========================================
  Files         185     185             
  Lines        8405    8407      +2     
========================================
- Hits         7893    3554   -4339     
- Misses        512    4853   +4341