Drop support for Ledger as a separate content type

Khoj will soon get a generic text indexing content type. This along
with a file filter should suffice for searching through Ledger
transactions, if required.

Having a specific content type for niche use-case like ledger isn't
useful. Removing unused content types will reduce khoj code to manage.

pyproject.toml

@@ -19,7 +19,6 @@ keywords = [
     "AI",
     "org-mode",
     "markdown",
-    "beancount",
     "images",
     "pdf",
 ]