Consider removing the validation for input location

[greyli]

In the current implementation, if the user uses an unsupported input location in app.input(location=...), APIFlask will raise a ValueError.

Pros:

• Allow users to create a custom location loader

Cons:

• No error is raised if the user uses a bad location (typo)

[mmdbalkhi]

Hello, I want to work on this, can you give me more information?

[greyli]

Just remove these lines and the related variable. Also, update tests and changelog.