Remove smithy-cli from tap_migrations.json

[haydenbaker]

Description of changes:

• When having both the aws and smithy-lang taps tapped, users will run into an issue where they have to fully qualify the brew command with smithy-lang/tap

  
  ❯ brew upgrade smithy-cli   
  ==> Downloading https://formulae.brew.sh/api/formula.jws.json

Warning: Formula aws/tap/smithy-cli was renamed to smithy-lang/tap/smithy-cli. Error: Formulae found in multiple taps:

• smithy-lang/tap/smithy-cli
• smithy-lang/tap/smithy-cli

Please use the fully-qualified name (e.g. smithy-lang/tap/smithy-cli) to refer to a specific formula.

- Removing this entry means this problem will go away
- I did not remove the README entry about the formula being migrated, since it is still valid
- Note: even untapping smithy-lang and keeping aws still renders the error - I'm not sure if this is a bug in how brew handles migrations (but it does not happen for formula migrated to homebrew/core

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