Polio-1708 delete deprecated endpoints

[quang-le]

What problem is this PR solving? Explain here in one sentence.

Related JIRA tickets : POLIO-1708

Self proofreading checklist

• Did I use eslint and black formatters
• Is my code clear enough and well documented
• Are my typescript files well typed
• New translations have been added or updated if new strings have been introduced in the frontend
• My migrations file are included
• Are there enough tests
• Documentation has been included (for new feature)

Doc

Added some comments

Changes

• deleted unused imstats and forma endpoints + deleted related tests

How to test

🤷‍♂️

Print screen / video

Not applicable

Notes

/polio/v2/forma/ will be rempoved in another PR as we need to move some logic to OpenHexa

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fix: empty instance pop up

Refs: IA-3665

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[mathvdh]

If we are sure they are not used anymore let's go