Remove set_weighted_stake extrtinsic and tests

Description

Remove add_weighted_stake for now because it creates too much slippage.

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation update
[ ] Other (please describe):

There should not be any impact because the removed feature was never released.

[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
[ ] I have made corresponding changes to the documentation
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules

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