fix: remove mdbook-lint preprocessor configuration

- Remove duplicate .mdbook-lint.toml from root (keep only in docs/)
- Remove mdbook-lint preprocessor from book.toml
- Will re-enable after upstream fixes per issue #37

Author: joshrotenberg

.mdbook-lint.toml

@@ -14,7 +14,3 @@ edit-url-template = "https://github.com/joshrotenberg/redisctl/edit/main/docs/{p
 [output.html.fold]
 enable = true
 level = 1
-
-# Testing mdbook-lint preprocessor config discovery
-[preprocessor.lint]
-command = "mdbook-lint"