Remove license-file from rust crates

#3866 added `license`.
Now building shows a warning:
> warning: /home/user/code/ruby-prism/rust/ruby-prism/Cargo.toml: only one of `license` or `license-file` is necessary
> `license` should be used if the package license can be expressed with a standard SPDX expression.
> `license-file` should be used if the package uses a non-standard license.

Author: Earlopain

rust/ruby-prism-sys/Cargo.toml

@@ -3,7 +3,6 @@ name = "ruby-prism-sys"
 version = "1.9.0"
 edition = "2021"
 license = "MIT"
-license-file = "../../LICENSE.md"
 repository = "https://github.com/ruby/prism"
 description = "Rust bindings to Ruby's prism parsing library"
 links = "prism"

rust/ruby-prism/Cargo.toml

@@ -3,7 +3,6 @@ name = "ruby-prism"
 version = "1.9.0"
 edition = "2021"
 license = "MIT"
-license-file = "../../LICENSE.md"
 repository = "https://github.com/ruby/prism"
 description = "Rustified version of Ruby's prism parsing library"
 authors = [