Remove the __private_api_log_lit special case

[ghost]

#445 (comment):

That special case existed to minimize the amount of code generated, but it's not compatible with format_args_capture as you pointed out. It should just be removed.

Closes #445.

I did not add a test for format_args_capture. I think I need to create a custom logger to capture records and add a testing-only crate feature or --cfg (to tell if #![feature(format_args_capture)] is allowed to use) to test it. format_args_capture is unstable now, I'm not sure if it worths it. Feel free to ask me to add such test if it's necessary.

[ghost]

The CI failure looks like rust-lang/cargo#8517.

    Updating crates.io index
warning: spurious network error (2 tries remaining): error inflating zlib stream; class=Zlib (5)
warning: spurious network error (1 tries remaining): error inflating zlib stream; class=Zlib (5)
error: failed to get `cfg-if` as a dependency of package `log v0.4.13 (D:\a\log\log)`

Caused by:
  failed to load source for dependency `cfg-if`

Caused by:
  Unable to update registry `https://github.com/rust-lang/crates.io-index`

Caused by:
  failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  error inflating zlib stream; class=Zlib (5)
Error: Process completed with exit code 1.